docs(13-03): document bug fixes found during verification

Added deviations section documenting 2 auto-fixed bugs:
1. ESLint no-unused-expressions (optional chaining)
2. Duplicate /api/ prefix in REST endpoint URL

Both bugs found and fixed during user verification (Task 4).

.planning/phases/13-file-upload-infrastructure/13-03-SUMMARY.md

@@ -91,12 +91,24 @@ completed: 2026-02-05
    - Imported and rendered JKChatUploadProgress component
    - Conditional rendering when isUploading && uploadFileName
 
-4. **Task 4: Human verification checkpoint** - APPROVED
+4. **Bug Fix: ESLint optional chaining errors** - `edf74f724` (fix)
+   - Found during user verification
+   - Replaced optional chaining with explicit if checks
+   - Modified: jam-ui/src/components/client/JKSessionScreen.js
+
+5. **Bug Fix: Duplicate /api/ prefix in endpoint URL** - `b1ed2247b` (fix)
+   - Found during user verification (backend 404 error)
+   - Removed /api/ prefix (already in REACT_APP_API_BASE_URL)
+   - Modified: jam-ui/src/helpers/rest.js
+
+6. **Task 4: Human verification checkpoint** - APPROVED
    - User confirmed: File dialog opens on button click
    - User confirmed: Invalid files show error toast (no upload)
    - User confirmed: Valid files trigger upload with progress shown in chat
    - User confirmed: Backend API returns 201 Created for valid uploads
 
+**Plan metadata:** `ff6042e88` (docs: complete plan)
+
 ## Files Created/Modified
 - `jam-ui/src/components/client/JKSessionScreen.js` - Attach button functionality with validation and upload dispatch
 - `jam-ui/src/components/client/chat/JKChatUploadProgress.js` - Upload progress indicator component (NEW)
@@ -130,7 +142,28 @@ completed: 2026-02-05
 
 ## Deviations from Plan
 
-None - plan executed exactly as written.
+### Auto-fixed Issues
+
+**1. [Rule 1 - Bug] ESLint no-unused-expressions errors**
+- **Found during:** User verification (Task 4)
+- **Issue:** Optional chaining expressions (attachFileInputRef.current?.click()) violated ESLint configuration
+- **Fix:** Replaced with explicit if checks: `if (attachFileInputRef.current) { attachFileInputRef.current.click(); }`
+- **Files modified:** jam-ui/src/components/client/JKSessionScreen.js
+- **Verification:** ESLint errors resolved, file dialog still opens correctly
+- **Committed in:** edf74f724 (separate fix commit during verification)
+
+**2. [Rule 1 - Bug] Duplicate /api/ prefix in REST endpoint URL**
+- **Found during:** User verification (Task 4) - backend returning 404
+- **Issue:** uploadMusicNotation used `/api/music_notations` but REACT_APP_API_BASE_URL already includes `/api`, resulting in `/api/api/music_notations`
+- **Fix:** Changed endpoint to `music_notations` (without /api/ prefix)
+- **Files modified:** jam-ui/src/helpers/rest.js
+- **Verification:** Backend returns 201 Created, upload succeeds
+- **Committed in:** b1ed2247b (separate fix commit during verification)
+
+---
+
+**Total deviations:** 2 auto-fixed bugs (both found during user verification)
+**Impact on plan:** Both bugs were correctness issues discovered during verification. Fixes were minimal (1-line changes) and did not affect scope.
 
 ## Issues Encountered