fix(install): self-chowning entrypoint kills fresh-install restart loop (#484)

The fresh-install restart loop reported by @MrGabri (and confirmed by
@AloePacci with the user:0:0 workaround) had a clear root cause:

  - Dockerfile pinned USER nodejs (UID 1001) before the entrypoint
    ran, so the existing chown branch in init-production.sh:13 was
    dead code.
  - wait-for-db.sh (the actual entrypoint, not init-production.sh)
    silently swallowed mkdir/EACCES on bind mounts with || true,
    then a downstream migration error surfaced as the visible failure.
  - Net effect on a typical Linux host where the bind-mount dir is
    owned by UID 1000: container can't write, exits non-zero,
    restarts forever with no clear error.

Switch to the standard Docker drop-privileges pattern:

  1. Install su-exec, drop `USER nodejs` from the Dockerfile —
     container now starts as root.
  2. wait-for-db.sh: if running as root, chown /app/storage,
     /app/data, /app/logs to nodejs and re-exec self via
     su-exec nodejs:nodejs. App still ends up running as UID 1001.
  3. Preflight check for non-root invocations (compose `user:`
     overrides): verify the bind mounts are actually writable
     before continuing. If not, exit 1 immediately with an
     actionable error pointing at the docs — no more silent
     restart loops.

Also:

  - Delete backend/init-production.sh. It was an orphan — no caller
    in the Dockerfile, compose, or anywhere else. Its chown logic
    looked authoritative enough that @MrGabri ran it manually trying
    to debug, which is what finally surfaced the EACCES.
  - docker-compose.yml: drop user: + PUID/PGID env. The pattern-B
    UID-matching workaround they implemented is obsolete now that
    pattern A (root-then-drop) is in place.
  - .env.example + README: drop PUID/PGID documentation.
  - Add fresh-install smoke test workflow. Boots backend + postgres
    against bind mounts owned by UID 1000 (the GitHub runner UID,
    and the common-mismatch case on Linux hosts) and verifies:
    + container reaches healthy without restart-looping
    + chown happened (dirs now owned by 1001 inside the container)
    + node runs as nodejs, not root (su-exec drop worked)
    + /health returns status:ok
    + with --user 5005:5005 + unwritable mounts, preflight exits
      loud with the expected error string

Verified locally end-to-end against a fresh Postgres + UID-501-owned
bind mount: backend reaches healthy in ~20s, chown applied, node
runs as nodejs, no restart loop. Docs in picpeak-docs cover the new
behavior + a Troubleshooting section for the install-path bugs
fixed in #484/#494/#511/#488.

Refs: #484

backend/Dockerfile

@@ -35,12 +35,15 @@ RUN apk upgrade --no-cache
 RUN npm install -g npm@10
 
 # Install dumb-init for proper signal handling, postgresql-client for database
-# checks, and ffmpeg for video upload support. Alpine's ffmpeg package ships
-# both `ffmpeg` and `ffprobe` built natively against musl libc — the npm
+# checks, ffmpeg for video upload support, and su-exec for the root → nodejs
+# privilege drop in wait-for-db.sh (see #484: container starts as root so it
+# can chown bind-mounted host volumes to UID 1001, then re-execs as nodejs
+# before running the app). Alpine's ffmpeg package ships both `ffmpeg` and
+# `ffprobe` built natively against musl libc — the npm
 # `@ffmpeg-installer/ffmpeg` binary is glibc-built and (a) doesn't reliably
 # run on Alpine and (b) only includes ffmpeg, not ffprobe (which the video
 # pipeline calls via fluent-ffmpeg.ffprobe()).
-RUN apk add --no-cache dumb-init postgresql-client ffmpeg
+RUN apk add --no-cache dumb-init postgresql-client ffmpeg su-exec
 
 # Create non-root user
 RUN addgroup -g 1001 -S nodejs && adduser -S nodejs -u 1001
@@ -56,7 +59,9 @@ RUN chmod -R a+r /app && chmod +x wait-for-db.sh
 RUN mkdir -p storage/events/active storage/events/archived storage/thumbnails data logs && \
     chown -R nodejs:nodejs storage data logs
 
-USER nodejs
+# No USER directive — the container starts as root so wait-for-db.sh can
+# chown bind-mounted host directories to UID 1001 before dropping privs
+# via su-exec. See #484 for the fresh-install restart loop this avoids.
 
 EXPOSE 3000