feat: map view with GPS extraction fix

Adds a new Map sidebar entry that plots photos by their EXIF GPS
coordinates on a clustered Leaflet map. While wiring this up, the
metadata extractor was reading unprefixed GPS keys that never exist
in `exiftool -G -j` output AND assumed coordinates were already
floats — every photo silently lost its GPS. The new extract_gps
helper handles Composite/EXIF group prefixes and parses DMS strings,
and lat/lon are stored as first-class indexed columns so the map
can query them without parsing exif_json on every request.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/photos.py

@@ -36,6 +36,8 @@ class PhotoResponse(PhotoBase):
     processing_status: str = 'pending'
     processing_error: Optional[str] = None
     exif_json: Optional[str] = None
+    latitude: Optional[float] = None
+    longitude: Optional[float] = None
     is_duplicate: bool = False
     live_photo_video_id: Optional[str] = None
     # tags: List[Dict[str, Any]] = []  # TODO: Enable when using eager loading