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CLAUDE.md

@@ -709,3 +709,100 @@ if (!hasLoTWConfirmation && hasDCLData && missingEntity) {
 - Clarified that DCL API doesn't send DXCC fields (current limitation)
 - Implemented priority logic: LoTW entity data takes precedence over DCL
 - System ready to auto-use DCL DXCC data if they add it in future API updates
+
+### Critical LoTW Sync Behavior (LEARNED THE HARD WAY)
+
+**⚠️ IMPORTANT: LoTW sync MUST only import confirmed QSOs**
+
+After attempting to implement "QSO Delta" sync (all QSOs, confirmed + unconfirmed), we discovered:
+
+**The Problem:**
+LoTW ADIF export with `qso_qsl=no` (all QSOs mode) only includes:
+- `CALL` (callsign)
+- `QSL_RCVD` (confirmation status: Y/N)
+
+**Missing Fields for Unconfirmed QSOs:**
+- `DXCC` (entity ID) ← **CRITICAL for awards!**
+- `COUNTRY` (entity name)
+- `CONTINENT`
+- `CQ_ZONE`
+- `ITU_ZONE`
+
+**Result:** Unconfirmed QSOs have `entityId: null` and `entity: ""`, breaking award calculations.
+
+**Current Implementation (CORRECT):**
+```javascript
+// lotw.service.js - fetchQSOsFromLoTW()
+const params = new URLSearchParams({
+  login: lotwUsername,
+  password: loTWPassword,
+  qso_query: '1',
+  qso_qsl: 'yes',  // ONLY confirmed QSOs
+  qso_qslsince: dateStr,  // Incremental sync
+});
+```
+
+**Why This Matters:**
+- Awards require `entityId` to count entities
+- Without `entityId`, QSOs can't be counted toward DXCC, WAS, etc.
+- Users can still see "worked" stations in QSO list, but awards only count confirmed
+- DCL sync can import all QSOs because it provides entity data via callsign lookup
+
+**Attempted Solution (REVERTED):**
+- Tried implementing callsign prefix lookup to populate missing `entityId`
+- Created `src/backend/utils/callsign-lookup.js` with basic prefix mappings
+- Complexity: 1000+ DXCC entities, many special event callsigns, portable designators
+- Decision: Too complex, reverted (commit 310b154)
+
+**Takeaway:** LoTW confirmed QSOs have reliable DXCC data. Don't try to workaround this fundamental limitation.
+
+### QSO Confirmation Filters
+
+Added "Confirmed by at least 1 service" filter to QSO view (commit 688b0fc):
+
+**Filter Options:**
+- "All QSOs" - No filter
+- "Confirmed by at least 1 service" (NEW) - LoTW OR DCL confirmed
+- "LoTW Only" - Confirmed by LoTW but NOT DCL
+- "DCL Only" - Confirmed by DCL but NOT LoTW
+- "Both Confirmed" - Confirmed by BOTH LoTW AND DCL
+- "Not Confirmed" - Confirmed by NEITHER
+
+**SQL Logic:**
+```sql
+-- "Confirmed by at least 1 service"
+WHERE lotwQslRstatus = 'Y' OR dclQslRstatus = 'Y'
+
+-- "LoTW Only"
+WHERE lotwQslRstatus = 'Y' AND (dclQslRstatus IS NULL OR dclQslRstatus != 'Y')
+
+-- "DCL Only"
+WHERE dclQslRstatus = 'Y' AND (lotwQslRstatus IS NULL OR lotwQslRstatus != 'Y')
+
+-- "Both Confirmed"
+WHERE lotwQslRstatus = 'Y' AND dclQslRstatus = 'Y'
+
+-- "Not Confirmed"
+WHERE (lotwQslRstatus IS NULL OR lotwQslRstatus != 'Y')
+AND (dclQslRstatus IS NULL OR dclQslRstatus != 'Y')
+```
+
+### Recent Development Work (January 2025)
+
+**Sync Type Support (ATTEMPTED & REVERTED):**
+- Commit 5b78935: Added LoTW sync type support (QSL/QSO delta/full)
+- Commit 310b154: Reverted - LoTW doesn't provide entity data for unconfirmed QSOs
+- **Lesson:** Keep it simple - only sync confirmed QSOs from LoTW
+
+**Dashboard Enhancements:**
+- Added sync job history display with real-time polling (every 2 seconds)
+- Shows job progress, status, and import logs
+- Cancel button for stale/failed jobs with rollback capability
+- Tracks all QSO changes in `qso_changes` table for rollback
+
+**Rollback System:**
+- `cancelJob(jobId, userId)` - Cancels and rolls back sync jobs
+- Tracks added QSOs (deletes them on rollback)
+- Tracks updated QSOs (restores previous state)
+- Only allows canceling failed jobs or stale running jobs (>1 hour)
+- Server-side validation prevents unauthorized cancellations