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/database.py

@@ -91,11 +91,18 @@ async def init_db():
                     "duplicate_group_id",
                     "ALTER TABLE photos ADD COLUMN duplicate_group_id VARCHAR",
                 ),
+                ("latitude", "ALTER TABLE photos ADD COLUMN latitude REAL"),
+                ("longitude", "ALTER TABLE photos ADD COLUMN longitude REAL"),
             ]
+            # Track whether the GPS columns were just added so we can kick
+            # off a one-shot backfill of existing photos at the end of init.
+            gps_columns_added = False
             for col_name, alter_sql in pending_alters:
                 if col_name not in existing_cols:
                     logger.info(f"Adding photos.{col_name} column")
                     await conn.execute(text(alter_sql))
+                    if col_name in ("latitude", "longitude"):
+                        gps_columns_added = True
             # Indexes for the new duplicate-detection columns. CREATE INDEX
             # IF NOT EXISTS is supported on SQLite so this is safe to run
             # every startup.
@@ -108,9 +115,27 @@ async def init_db():
                     "ON photos(duplicate_group_id)"
                 )
             )
+            await conn.execute(
+                text(
+                    "CREATE INDEX IF NOT EXISTS ix_photos_lat_lon "
+                    "ON photos(latitude, longitude)"
+                )
+            )
 
         logger.info("Database initialized successfully")
 
+        # If we just introduced the GPS columns on an existing install, kick
+        # off a one-shot backfill so the Map view is populated without a
+        # manual full re-scan. Imported lazily to avoid pulling Celery into
+        # the import graph for non-worker processes that don't need it.
+        if "sqlite" in settings.database_url and gps_columns_added:
+            try:
+                from app.tasks.scan import backfill_gps
+                backfill_gps.delay()
+                logger.info("Queued one-shot backfill_gps task after column add")
+            except Exception as e:
+                logger.warning(f"Could not queue backfill_gps task: {e}")
+
 async def create_fts_table():
     """Create Full-Text Search table for SQLite"""
     if "sqlite" in settings.database_url: