fix: enable debug logging and improve DCL sync observability

- Fix logger bug where debug level (0) was treated as falsy
  - Change `||` to `??` in config.js to properly handle log level 0
  - Debug logs now work correctly when LOG_LEVEL=debug

- Add server startup logging
  - Log port, environment, and log level on server start
  - Helps verify configuration is loaded correctly

- Add DCL API request debug logging
  - Log full API request parameters when LOG_LEVEL=debug
  - API key is redacted (shows first/last 4 chars only)
  - Helps troubleshoot DCL sync issues

- Update CLAUDE.md documentation
  - Add Logging section with log levels and configuration
  - Document debug logging feature for DCL service
  - Add this fix to Recent Commits section

Note: .env file added locally with LOG_LEVEL=debug (not committed)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -19,6 +19,22 @@ Default to using Bun instead of Node.js.
 - Prefer `Bun.file` over `node:fs`'s readFile/writeFile
 - Bun.$`ls` instead of execa.
 
+## Logging
+
+The application uses a custom logger in `src/backend/config.js`:
+- **Log levels**: `debug` (0), `info` (1), `warn` (2), `error` (3)
+- **Default**: `debug` in development, `info` in production
+- **Override**: Set `LOG_LEVEL` environment variable (e.g., `LOG_LEVEL=debug`)
+- **Output format**: `[timestamp] LEVEL: message` with JSON data
+
+**Important**: The logger uses the nullish coalescing operator (`??`) to handle log levels. This ensures that `debug` (level 0) is not treated as falsy.
+
+Example `.env` file:
+```
+NODE_ENV=development
+LOG_LEVEL=debug
+```
+
 ## Testing
 
 Use `bun test` to run tests.
@@ -170,12 +186,33 @@ The award system is JSON-driven and located in `award-definitions/` directory. E
 - `normalizeMode(mode)`: Standardize mode names (CW, FT8, SSB, etc.)
 - Used by both LoTW and DCL services for consistency
 
+**Job Queue Service**: `src/backend/services/job-queue.service.js`
+- Manages async background jobs for LoTW and DCL sync
+- `enqueueJob(userId, jobType)`: Queue a sync job ('lotw_sync' or 'dcl_sync')
+- `processJobAsync(jobId, userId, jobType)`: Process job asynchronously
+- `getUserActiveJob(userId, jobType)`: Get active job for user (optional type filter)
+- `getJobStatus(jobId)`: Get job status with parsed result
+- `updateJobProgress(jobId, progressData)`: Update job progress during processing
+- Supports concurrent LoTW and DCL sync jobs
+- Job types: 'lotw_sync', 'dcl_sync'
+- Job status: 'pending', 'running', 'completed', 'failed'
+
+**Backend API Routes** (`src/backend/index.js`):
+- `POST /api/lotw/sync`: Queue LoTW sync job
+- `POST /api/dcl/sync`: Queue DCL sync job
+- `GET /api/jobs/:jobId`: Get job status
+- `GET /api/jobs/active`: Get active job for current user
+
 **DCL Service**: `src/backend/services/dcl.service.js`
 - `fetchQSOsFromDCL(dclApiKey, sinceDate)`: Fetch from DCL API
+- API Endpoint: `https://dings.dcl.darc.de/api/adiexport`
+- Request: POST with JSON body `{ key, limit: 50000, qsl_since, qso_since, cnf_only }`
 - `parseDCLJSONResponse(jsonResponse)`: Parse example/test payloads
 - `syncQSOs(userId, dclApiKey, sinceDate, jobId)`: Sync QSOs to database
 - `getLastDCLQSLDate(userId)`: Get last QSL date for incremental sync
-- Ready for when DCL publishes their API
+- Debug logging (when `LOG_LEVEL=debug`) shows API params with redacted key (first/last 4 chars)
+- Fully implemented and functional
+- **Note**: DCL API is a custom prototype by DARC; contact DARC for API specification details
 
 ### DLD Award Implementation (COMPLETED)
 
@@ -214,6 +251,20 @@ The DLD (Deutschland Diplom) award was recently implemented:
 
 **Documentation**: See `docs/DOCUMENTATION.md` for complete documentation including DLD award example.
 
+**Frontend**: `src/frontend/src/routes/qsos/+page.svelte`
+- Separate sync buttons for LoTW (blue) and DCL (orange)
+- Independent progress tracking for each sync type
+- Both syncs can run simultaneously
+- Job polling every 2 seconds for status updates
+- Import log displays after sync completion
+- Real-time QSO table refresh after sync
+
+**Frontend API** (`src/frontend/src/lib/api.js`):
+- `qsosAPI.syncFromLoTW()`: Trigger LoTW sync
+- `qsosAPI.syncFromDCL()`: Trigger DCL sync
+- `jobsAPI.getStatus(jobId)`: Poll job status
+- `jobsAPI.getActive()`: Get active job on page load
+
 ### Adding New Awards
 
 To add a new award:
@@ -230,20 +281,23 @@ To add a new award:
 
 - **LoTW (Logbook of The World)**: ARRL's confirmation system
   - Service: `src/backend/services/lotw.service.js`
+  - API: `https://lotw.arrl.org/lotwuser/lotwreport.adi`
   - Fields: `lotwQslRstatus`, `lotwQslRdate`
   - Used for DXCC, WAS, VUCC, most awards
   - ADIF format with `<EOR>` delimiters
-  - Supports incremental sync by date
+  - Supports incremental sync by `qso_qslsince` parameter (format: YYYY-MM-DD)
 
 - **DCL (DARC Community Logbook)**: DARC's confirmation system
   - Service: `src/backend/services/dcl.service.js`
+  - API: `https://dings.dcl.darc.de/api/adiexport`
   - Fields: `dclQslRstatus`, `dclQslRdate`
   - DOK fields: `darcDok` (partner's DOK), `myDarcDok` (user's DOK)
   - Required for DLD award
   - German amateur radio specific
-  - API in development (parser ready)
+  - Request format: POST JSON `{ key, limit, qsl_since, qso_since, cnf_only }`
   - Response format: JSON with ADIF string in `adif` field
-  - Supports DOK (DARC Ortsverband Kennung) data
+  - Supports incremental sync by `qsl_since` parameter (format: YYYYMMDD)
+  - Updates QSOs only if confirmation data has changed
 
 ### ADIF Format
 
@@ -262,10 +316,50 @@ Both LoTW and DCL return data in ADIF (Amateur Data Interchange Format):
 
 ### Recent Commits
 
+- **Uncommitted**: fix: logger debug level not working
+  - Fixed bug where debug logs weren't showing due to falsy value handling
+  - Changed `||` to `??` in logger config to properly handle log level 0 (debug)
+  - Added `.env` file with `LOG_LEVEL=debug` for development
+  - Debug logs now show DCL API request parameters with redacted API key
+- `27d2ef1`: fix: preserve DOK data when DCL doesn't send values
+  - DCL sync only updates DOK/grid fields when DCL provides non-empty values
+  - Prevents accidentally clearing DOK data from manual entry or other sources
+  - Preserves existing DOK when DCL syncs QSO without DOK information
+- `e09ab94`: feat: skip QSOs with unchanged confirmation data
+  - LoTW/DCL sync only updates QSOs if confirmation data has changed
+  - Tracks added, updated, and skipped QSO counts
+  - LoTW: Checks if lotwQslRstatus or lotwQslRdate changed
+  - DCL: Checks if dclQslRstatus, dclQslRdate, darcDok, myDarcDok, or grid changed
+- `3592dbb`: feat: add import log showing synced QSOs
+  - Backend returns addedQSOs and updatedQSOs arrays in sync result
+  - Frontend displays import log with callsign, date, band, mode for each QSO
+  - Separate sections for "New QSOs" and "Updated QSOs"
+  - Sync summary shows total, added, updated, skipped counts
 - `8a1a580`: feat: implement DCL ADIF parser and service integration
   - Add shared ADIF parser utility (src/backend/utils/adif-parser.js)
-  - Implement DCL service with API integration ready
+  - Implement DCL service with API integration
   - Refactor LoTW service to use shared parser
   - Tested with example DCL payload (6 QSOs parsed successfully)
 - `c982dcd`: feat: implement DLD (Deutschland Diplom) award
 - `322ccaf`: docs: add DLD (Deutschland Diplom) award documentation
+
+### Sync Behavior
+
+**Import Log**: After each sync, displays a table showing:
+- New QSOs: Callsign, Date, Band, Mode
+- Updated QSOs: Callsign, Date, Band, Mode (only if data changed)
+- Skipped QSOs: Counted but not shown (data unchanged)
+
+**Duplicate Handling**:
+- QSOs matched by: userId, callsign, qsoDate, timeOn, band, mode
+- If confirmation data unchanged: Skipped (not updated)
+- If confirmation data changed: Updated with new values
+- Prevents unnecessary database writes and shows accurate import counts
+
+**DOK Update Behavior**:
+- If QSO imported via LoTW (no DOK) and later DCL confirms with DOK: DOK is added ✓
+- If QSO already has DOK and DCL sends different DOK: DOK is updated ✓
+- If QSO has DOK and DCL syncs without DOK (empty): Existing DOK is preserved ✓
+- LoTW never sends DOK data; only DCL provides DOK fields
+
+**Important**: DCL sync only updates DOK/grid fields when DCL provides non-empty values. This prevents accidentally clearing DOK data that was manually entered or imported from other sources.