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/routers/photos.py

@@ -214,6 +214,36 @@ async def list_photos(
         "pages": (total + per_page - 1) // per_page if total else 0,
     }
 
+@router.get("/map")
+async def list_photos_with_gps(db: AsyncSession = Depends(get_db)):
+    """Lightweight listing of every non-discarded photo that has GPS
+    coordinates, used by the Map view. Intentionally returns a flat list
+    (no pagination) with only the fields the map renderer needs, so even
+    large libraries serialize to a few MB at most. Declared *before*
+    ``/{photo_id}`` so the literal path wins the FastAPI route match."""
+    result = await db.execute(
+        select(
+            Photo.id,
+            Photo.latitude,
+            Photo.longitude,
+            Photo.taken_at,
+        ).where(
+            Photo.is_discarded.is_(False),
+            Photo.latitude.is_not(None),
+            Photo.longitude.is_not(None),
+        )
+    )
+    return [
+        {
+            "id": row.id,
+            "latitude": row.latitude,
+            "longitude": row.longitude,
+            "taken_at": row.taken_at.isoformat() if row.taken_at else None,
+        }
+        for row in result.all()
+    ]
+
+
 @router.get("/{photo_id}")
 async def get_photo(
     photo_id: str,