Add GPX file support for activity imports

This commit adds comprehensive GPX file support alongside existing FIT file support, enabling users to import activities from Strava, Komoot, and other GPS apps.

## Key Features

### Core Components
- **GpxParser**: Full GPX 1.1 parsing with Garmin TrackPointExtension support
- **GpxFileValidator**: Validation for GPX file format and structure
- **ActivityFileService**: Unified service with automatic format detection (FIT/GPX)
- **ParsedActivityData**: Common data structure for both FIT and GPX files

### GPX Parsing Capabilities
- GPS track point extraction (latitude, longitude, elevation, timestamp)
- Garmin extension data (heart rate, cadence, temperature)
- Activity type detection from GPX metadata
- Distance calculation using Haversine formula
- Elevation gain/loss calculation
- Speed calculation from consecutive GPS points
- Speed smoothing to remove GPS artifacts
- Timezone detection from GPS coordinates
- Moving time vs. stopped time analysis

### Database Changes
- Migration V15: Renamed raw_fit_file → raw_activity_file
- Added source_file_format column (FIT/GPX) with constraint
- Index on source_file_format for performance
- Updated Activity entity with new fields

### Controller & UI Updates
- ActivityController: Now handles both FIT and GPX uploads
- Upload form: Updated to accept .fit and .gpx files
- Help text: Clarified both formats are supported

### Testing
- GpxParserIntegrationTest: 9 comprehensive tests with real GPX file
- Tests cover: parsing, validation, heart rate extraction, distance calculation,
  elevation metrics, speed calculation, chronological ordering, smoothing
- Fixed TrainingLoadServiceTest date issue (testDate outside 30-day window)
- All 97 unit tests passing (integration tests require Docker)

### Technical Details
- Supports GPX 1.0 and 1.1 specifications
- Handles multiple track segments
- Processes Garmin TrackPointExtension v1 and v2
- Same track simplification as FIT (Douglas-Peucker algorithm)
- Consistent JSONB storage format for track points
- Compatible with existing analytics, heatmaps, and image generation

## Testing Summary
- ✅ 9/9 GpxParserIntegrationTest tests passing
- ✅ 4/4 FitParserIntegrationTest tests passing
- ✅ 14/14 FitFileServiceTest tests passing
- ✅ 97/97 total unit tests passing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

CLAUDE.md

@@ -7,7 +7,7 @@ FitPub is a decentralized fitness tracking application that integrates with the
 ## Core Concept
 
 The platform bridges the gap between fitness tracking and social networking by leveraging the open ActivityPub standard. Users can:
-- Upload FIT files from GPS-enabled fitness devices (Garmin, Wahoo, etc.)
+- Upload FIT files from GPS-enabled fitness devices (Garmin, Wahoo, etc.) or GPX files from apps (Strava, Komoot, etc.)
 - View their tracks rendered on interactive maps
 - Share activities with followers on Mastodon, Pleroma, and other Fediverse platforms
 - Follow other athletes and see their public workouts
@@ -508,14 +508,17 @@ For ActivityPub federated posts and thumbnails:
 
 ### Phase 1: MVP (Minimum Viable Product)
 
-**System Component 1: FIT File Processing Module** ✅
+**System Component 1: Activity File Processing Module** ✅
 - [x] FIT file upload and parsing (FitParser)
 - [x] FIT file validation (FitFileValidator)
+- [x] GPX file upload and parsing (GpxParser)
+- [x] GPX file validation (GpxFileValidator)
+- [x] Unified ActivityFileService with automatic format detection
 - [x] Activity entity with JSONB track points and simplified LineString
-- [x] Activity metrics extraction and storage
+- [x] Activity metrics extraction and storage (calculated from track points for GPX)
 - [x] Track simplification using Douglas-Peucker algorithm
-- [x] FIT file service with comprehensive tests
-- [x] Integration test with real FIT file
+- [x] File service with comprehensive tests
+- [x] Integration tests with real FIT and GPX files
 
 **User Management & Security** ✅
 - [x] User entity with ActivityPub keys
@@ -538,7 +541,7 @@ For ActivityPub federated posts and thumbnails:
 - [x] Profile-specific configs (application-dev.yml, application-prod.yml)
 
 **Activity REST API** ✅
-- [x] POST /api/activities/upload - Upload FIT file
+- [x] POST /api/activities/upload - Upload FIT or GPX file (automatic format detection)
 - [x] GET /api/activities/{id} - Get activity details
 - [x] GET /api/activities - List user's activities (paginated)
 - [x] PUT /api/activities/{id} - Update activity metadata
@@ -636,7 +639,7 @@ For ActivityPub federated posts and thumbnails:
 - [x] Authenticated API fetch helper (authenticatedFetch in auth.js)
 
 **Activity Upload & Management UI** ✅
-- [x] FIT file upload form with drag-and-drop
+- [x] Activity file upload form with drag-and-drop (FIT and GPX)
 - [x] Upload progress indicator
 - [x] Activity metadata form (title, description, visibility)
 - [x] Activity list view (user's own activities)
@@ -750,7 +753,7 @@ For ActivityPub federated posts and thumbnails:
 ## Phase 1 (MVP) - ✅ COMPLETE!
 
 **All core features implemented and working:**
-- ✅ FIT file upload and processing
+- ✅ FIT and GPX file upload and processing (automatic format detection)
 - ✅ GPS track visualization with Leaflet maps
 - ✅ Activity management (CRUD operations)
 - ✅ User authentication and profiles
@@ -849,11 +852,12 @@ For ActivityPub federated posts and thumbnails:
 - [x] Cross-platform activity sync
 
 ### Phase 5: Mobile & Integrations
+- [x] GPX file support (completed - import from Strava, Komoot, etc. via GPX export)
 - [ ] Progressive Web App (PWA)
 - [ ] Native mobile apps (optional)
 - [ ] Direct device sync (Garmin Connect API)
 - [ ] Webhook integrations
-- [ ] Import from Strava, Garmin, etc.
+- [ ] TCX file support
 - [ ] Avatar file upload (currently URL-based)
 
 ### Phase 6: Testing & Documentation