build(sidecar): containerize + delete the Node prototype

- New sidecar/Dockerfile: multi-stage golang:1.25-alpine → distroless/
  static, ~12 MB final image, static CGO-free binary.
- Wire pp-sidecar into docker-compose.photoprism.yml so the whole
  stack (mariadb + photoprism + sidecar) starts with one
  `podman-compose up`. Container reaches mariadb + photoprism on the
  internal network; the host gets 127.0.0.1:8000 for Vite's proxy.
- New SIDECAR_LISTEN_ADDR env var (default 127.0.0.1 for the host-mode
  dev loop) so the container can bind 0.0.0.0:8000 and let the port
  mapping reach it. Without this the loopback bind was invisible to
  the host.
- Delete sidecar/legacy/server.mjs — the Node prototype's archival
  window is over; git history is its home now.
- Update sidecar/README with compose-first bringup; keep the host
  `go build` flow as the fast-iteration loop.

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

sidecar/README.md

@@ -31,7 +31,27 @@ Marks persist to **MariaDB** (`mule_sidecar.marks`); everything else
 operates on the filesystem under `${ORIGINALS_ROOT}` and triggers a
 PhotoPrism reindex of the affected parent in the background.
 
-## Build & run
+## Run
+
+The sidecar is a `sidecar` service in the PhotoPrism compose stack.
+Bringing the whole stack up brings it up too:
+
+```sh
+podman-compose --env-file .env.photoprism \
+  -f docker-compose.photoprism.yml \
+  -f docker-compose.photoprism.podman.yml \
+  up -d
+```
+
+This builds [Dockerfile](Dockerfile) (multi-stage `golang:1.25-alpine` →
+`gcr.io/distroless/static`, ~12 MB final image), starts the container,
+and binds `127.0.0.1:8000` to the service. The SvelteKit dev server
+proxies `/api/sidecar/*` to that port transparently.
+
+### Dev-iteration loop (host build)
+
+For tight iteration without rebuilding the image on every change you
+can run it as a host process — Go is already on the dev machine:
 
 ```sh
 cd sidecar
@@ -43,8 +63,8 @@ SIDECAR_PORT=8000 \
 ./mule-sidecar
 ```
 
-The SvelteKit dev server proxies `/api/sidecar/*` to
-`http://localhost:8000`.
+The host build connects to `mariadb` via the loopback port the compose
+file publishes; stop `pp-sidecar` first so they don't fight for 8000.
 
 ## Env
 
@@ -81,6 +101,7 @@ state. Heap-sharing tables (M4) will land in subsequent migrations.
 
 ```text
 sidecar/
+├── Dockerfile            multi-stage golang:1.25 → distroless/static
 ├── main.go               entrypoint, route wiring, graceful shutdown
 ├── config.go             env-driven Config
 ├── db.go                 GORM open + Mark model + AutoMigrate
@@ -91,6 +112,5 @@ sidecar/
 ├── handlers_folders.go
 ├── handlers_marks.go
 ├── handlers_heap.go
-├── handlers_dups.go
-└── legacy/server.mjs     Node prototype, retained for one cycle as a reference.
+└── handlers_dups.go
 ```