feat(sidecar): port Node prototype to Go + Gin + GORM + MariaDB

Replace the Node prototype (server.mjs) with the stack the merge plan
calls for: Go 1.25, Gin for routing, GORM + MariaDB for persistence.
Same wire contract on /api/sidecar/* so the SvelteKit client doesn't
change.

- Marks move from a JSON file on disk to mule_sidecar.marks (auto-
  migrated by GORM on first boot). The Node prototype's marks.json
  was dev-only; not migrated.
- Folder/rename/heap-convert/duplicates handlers reproduce the
  prototype's behaviour, including the path-traversal defence
  (resolveUnderRoot + EvalSymlinks), the size-bucket prefilter for
  the duplicate hasher, and the background reindex fire-and-forget
  pattern.
- Auth model unchanged: requireSession middleware proxies the
  caller's X-Auth-Token to PhotoPrism's /api/v1/photos?count=1
  before any destructive op.
- Expose pp-mariadb on 127.0.0.1:3306 in docker-compose so the
  host Go process can reach mule_sidecar.* without joining the
  container network.
- Archive the Node prototype under sidecar/legacy/server.mjs for
  one cycle as reference.

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

docker-compose.photoprism.yml

@@ -33,6 +33,12 @@ services:
       MARIADB_USER: ${PP_DB_USER:-photoprism}
       MARIADB_PASSWORD: ${PP_DB_PASSWORD:?set PP_DB_PASSWORD in .env.photoprism}
       MARIADB_ROOT_PASSWORD: ${PP_DB_ROOT_PASSWORD:?set PP_DB_ROOT_PASSWORD in .env.photoprism}
+    # Loopback-only host port so the mule-sidecar (running as a host process
+    # in M4) can reach `mule_sidecar.*` over TCP. Not exposed beyond
+    # 127.0.0.1; the photoprism container still resolves mariadb by service
+    # name on the photoprism-network bridge.
+    ports:
+      - "127.0.0.1:${PP_DB_PORT:-3306}:3306"
     volumes:
       - pp_mariadb_data:/var/lib/mysql
       # The init script creates the mule_sidecar database + user that the Go