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>

frontend/package.json

@@ -39,6 +39,7 @@
     "react-hotkeys-hook": "^4.4.3",
     "react-intersection-observer": "^9.5.3",
     "react-leaflet": "^4.2.1",
+    "react-leaflet-cluster": "^2.1.0",
     "tailwind-merge": "^2.2.0",
     "zustand": "^4.4.7"
   },