Phase 3: Go hygiene pass, runtime PUID/PGID, fork README

Go:
- staticcheck 399 -> 0 with staticcheck.conf (style checks ST1000/1003/
  1005/1016/1020/1021/1022 excluded; error strings are shown in the UI).
- io/ioutil and rand.Seed removed; CloseNotifier kept with a lint-ignore
  until the Phase 2 context rewrite.
- Dead code deleted: Auto handler, getStreamByChannelID, updateXEPG,
  indexOfInt, jsonToMapInt64, removeOldSystemData, randomTime, and the
  commented-out blocks in struct-buffer.go and internal/authentication.
- Duplicates folded: cacheImagesInBackground(), one addErrorToStream().
- Bugs found by SA4006/SA5001: os.Create handle leaked per ffmpeg segment
  (buffer.go), http.NewRequest error unchecked (buffer.go), xepg.json
  migration wrote null on read error (migrate.go), WriteUserData errors
  silently dropped (authentication.go), defer Close before error check
  (buffer.go, toolchain.go). checkFilePermission results were discarded;
  an unwritable config or temp dir is now fatal at start-up.
- gofmt applied repo-wide; Drone runs gofmt check and staticcheck.

Docker:
- Entrypoint starts as root, applies PUID/PGID (falls back to XTEVE_UID/
  XTEVE_GID, then image defaults), fixes config ownership only when it
  differs, then drops to xteve via su-exec. --user starts skip all of it.
- /xteve removed from LEGACY_CONFIG_DIRS (it is the parent of the default).
- mwader/static-ffmpeg pinned to 7.1.1; VOLUME /xteve/config.
- Compose files pull registry.coadcorp.com/nathan/xteve:latest, use
  PUID/PGID/TZ, and explain that SSDP needs host networking.
- .dockerignore excludes the npm toolchain (bundle stays in html/js).

Docs: README rewritten for the fork (about, registry, compose, env vars,
security notes); README-DEV gains a container section.

README-DEV.md

@@ -55,3 +55,7 @@ Drone (`.drone.yml`) runs vet, tests, the bundle check, hadolint and compose val
 ## Endpoints useful for operations
 
 * `GET /healthz` returns `{"status":"ok","version":...,"scanInProgress":...}` with no authentication. The Docker healthcheck uses it.
+
+## Container image
+
+`Dockerfile` builds the binary in a `golang:1.27.1-alpine` stage, copies static `ffmpeg`/`ffprobe` from a pinned `mwader/static-ffmpeg` tag (bump the tag in the Dockerfile; there is no other place to change) and runs on `alpine`. The runtime stage installs `su-exec` and does not set `USER`: `docker/entrypoint.sh` starts as root, applies `PUID`/`PGID` to the `xteve` user by editing `/etc/passwd` and `/etc/group`, fixes ownership of the config directory, and execs `su-exec xteve:xteve xteve`. With `--user` it skips all of that. `ARG XTEVE_UID`/`XTEVE_GID` remain the build-time defaults so Drone's build args still work. Check the script with `sh -n` and `shellcheck -s sh` after editing.