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/config.go

@@ -12,7 +12,8 @@ import (
 type Config struct {
 	OriginalsRoot     string // absolute path to PhotoPrism's originals dir
 	PhotoprismBaseURL string // e.g. http://localhost:2342
-	Port              int    // HTTP listen port (loopback only)
+	ListenAddr        string // bind interface — 127.0.0.1 for host mode, 0.0.0.0 in containers
+	Port              int    // HTTP listen port
 	DSN               string // GORM/MySQL connection string for mule_sidecar
 }
 
@@ -47,6 +48,7 @@ func loadConfig() (*Config, error) {
 	return &Config{
 		OriginalsRoot:     abs,
 		PhotoprismBaseURL: envOr("PHOTOPRISM_BASE_URL", "http://localhost:2342"),
+		ListenAddr:        envOr("SIDECAR_LISTEN_ADDR", "127.0.0.1"),
 		Port:              port,
 		DSN:               dsn,
 	}, nil