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>

sidecar/README.md

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+# mule-sidecar
+
+Auxiliary service that handles operations PhotoPrism's REST API does not expose.
+
+## Why this exists
+
+Per the merge plan at `/home/dtoro/.claude/plans/i-want-you-to-twinkly-galaxy.md`,
+a Go + Gin + GORM service (matching PhotoPrism's stack) will eventually own:
+
+- Per-user heap sharing with pending invitations
+- Folder mutations under `originals/` (create / rename / delete / move)
+- **File rename** on disk (PhotoPrism's `OriginalName` is a display-only rename)
+
+The plan picks Go for stack consistency and the option to upstream features.
+
+## What ships today
+
+A **Node.js prototype** (`server.mjs`) covering only the **file rename** endpoint.
+
+The decision to ship Node first is pragmatic — Go isn't installed on this dev
+box and `sudo dnf install golang` needs a password. Node is already on PATH for
+the SvelteKit dev server, so a single-file Node service unblocks the feature
+without changing the host setup.
+
+The endpoint contract is stable: when M4 lands the proper Go service, the
+SvelteKit client keeps calling the same paths.
+
+## Endpoints
+
+- `POST /api/sidecar/files/:photoUid/rename` `{ "newName": "newfile.png" }`
+  Renames the primary file of the photo on disk under `${ORIGINALS_ROOT}`,
+  then triggers a PhotoPrism reindex of the parent path.
+
+## Run
+
+```
+ORIGINALS_ROOT=/home/dtoro/projects/mule-image/photos-sample \
+PHOTOPRISM_BASE_URL=http://localhost:2342 \
+SIDECAR_PORT=8000 \
+node server.mjs
+```
+
+The SvelteKit dev server proxies `/api/sidecar/*` to `http://localhost:8000`.