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/models/photos.py

@@ -1,7 +1,7 @@
 """
 Photo model definition
 """
-from sqlalchemy import Column, String, Integer, Boolean, DateTime, ForeignKey, Text, Index
+from sqlalchemy import Column, String, Integer, Float, Boolean, DateTime, ForeignKey, Text, Index
 from sqlalchemy.sql import func
 from datetime import datetime
 import uuid
@@ -49,6 +49,13 @@ class Photo(Base):
     
     # Metadata
     exif_json = Column(Text)  # full EXIF/XMP blob as JSON
+
+    # GPS coordinates extracted from EXIF, in signed decimal degrees
+    # (S latitude / W longitude are negative). Stored as first-class columns
+    # so the Map view and any future location filters can query/index them
+    # without parsing exif_json on every request.
+    latitude = Column(Float)
+    longitude = Column(Float)
     
     # User-editable fields
     user_title = Column(String)
@@ -91,4 +98,5 @@ class Photo(Base):
         Index('ix_photos_color_label', 'color_label'),
         Index('ix_photos_media_type', 'media_type'),
         Index('ix_photos_processing_status', 'processing_status'),
+        Index('ix_photos_lat_lon', 'latitude', 'longitude'),
     )