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-lock.json

@@ -37,6 +37,7 @@
         "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"
       },
@@ -4572,6 +4573,15 @@
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       "license": "BSD-2-Clause"
     },
+    "node_modules/leaflet.markercluster": {
+      "version": "1.5.3",
+      "resolved": "https://registry.npmjs.org/leaflet.markercluster/-/leaflet.markercluster-1.5.3.tgz",
+      "integrity": "sha512-vPTw/Bndq7eQHjLBVlWpnGeLa3t+3zGiuM7fJwCkiMFq+nmRuG3RI3f7f4N4TDX7T4NpbAXpR2+NTRSEGfCSeA==",
+      "license": "MIT",
+      "peerDependencies": {
+        "leaflet": "^1.3.1"
+      }
+    },
     "node_modules/levn": {
       "version": "0.4.1",
       "resolved": "https://registry.npmjs.org/levn/-/levn-0.4.1.tgz",
@@ -5249,6 +5259,21 @@
         "react-dom": "^18.0.0"
       }
     },
+    "node_modules/react-leaflet-cluster": {
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/react-leaflet-cluster/-/react-leaflet-cluster-2.1.0.tgz",
+      "integrity": "sha512-16X7XQpRThQFC4PH4OpXHimGg19ouWmjxjtpxOeBKpvERSvIRqTx7fvhTwkEPNMFTQ8zTfddz6fRTUmUEQul7g==",
+      "license": "SEE LICENSE IN <LICENSE>",
+      "dependencies": {
+        "leaflet.markercluster": "^1.5.3"
+      },
+      "peerDependencies": {
+        "leaflet": "^1.8.0",
+        "react": "^18.0.0",
+        "react-dom": "^18.0.0",
+        "react-leaflet": "^4.0.0"
+      }
+    },
     "node_modules/react-refresh": {
       "version": "0.17.0",
       "resolved": "https://registry.npmjs.org/react-refresh/-/react-refresh-0.17.0.tgz",