feat: PhotoPrism M0 bring-up — compose stack, web client, sidecar, migrate

Replace the legacy mule-image backend with PhotoPrism plus a thin
SvelteKit client and a Node sidecar for endpoints PhotoPrism doesn't
expose (file rename), and add a two-phase migrator (metadata via PUT,
heaps → albums) for the existing Postgres library.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrate/example-snapshot.json

@@ -0,0 +1,36 @@
+{
+  "_comment": "Synthetic snapshot exercising the migrator against ./photos-sample. Run from repo root: node migrate/run.mjs --snapshot migrate/example-snapshot.json",
+  "photos": [
+    {
+      "filepath": "Screenshot_20260507_204050.png",
+      "user_title": "Paris dawn",
+      "user_notes": "Migration smoke test caption",
+      "rating": 4,
+      "is_picked": true,
+      "is_discarded": false,
+      "is_hidden": false,
+      "taken_at": "2024-06-15T07:00:00Z",
+      "tags": ["paris", "migration-test"]
+    },
+    {
+      "filepath": "Screenshot_20260508_000706.png",
+      "user_title": null,
+      "user_notes": "Another mule-image note",
+      "rating": null,
+      "is_picked": false,
+      "is_discarded": false,
+      "is_hidden": false,
+      "taken_at": "2024-07-04T18:30:00Z",
+      "tags": ["fireworks"]
+    }
+  ],
+  "heaps": [
+    {
+      "name": "M5 migration test",
+      "photo_filepaths": [
+        "Screenshot_20260507_204050.png",
+        "Screenshot_20260508_000706.png"
+      ]
+    }
+  ]
+}