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.

tasks/todo.md

@@ -46,13 +46,13 @@ Status: Phase 0 committed on branch `improvements` 2026-09-26.
 - [x] `//go:embed` via `html/embed.go`; deleted `webUI.go`, `html-build.go`, `cmd/webui-gen`; ETag + Cache-Control on static assets
 - [x] i18n dropped: 254 placeholders inlined, `en.json` deleted, only HTML pages templated (`authenticationErr`)
 - [x] `package.json` + `ts/tsconfig.json` (tsc 5.9.3, ES2020, single outFile `html/js/app.js`, committed, CI-verified); 10 dead JS files deleted; 6 tsc errors fixed incl. a real ASI bug in the search shortcut
-- [ ] CI: vet (done), lint, gofmt, `test -race`; bundle freshness check (done)
+- [x] CI: vet, gofmt check, staticcheck v0.8.1 (config in `staticcheck.conf`), bundle freshness check. `go test -race` still to add once tests exist
 - [ ] Version-tagged images (amd64 only)
-- [ ] PUID/PGID via su-exec; pin static-ffmpeg; `VOLUME`; fix `LEGACY_CONFIG_DIRS`
+- [x] PUID/PGID via su-exec at runtime; static-ffmpeg pinned to 7.1.1; `VOLUME /xteve/config`; `/xteve` removed from `LEGACY_CONFIG_DIRS`; compose files pull from the registry and document SSDP/host networking
 - [x] `/healthz` endpoint; Dockerfile healthcheck uses it
-- [ ] `README-DEV.md` (done) and fork section in `README.md`
-- [ ] Deduplicate and delete dead code; drop `ioutil`, `osext`, `rand.Seed` (after go:embed)
-- [ ] Resolve missing `docs/design-system/` referenced by `agent.md`
+- [x] `README-DEV.md` and fork-specific `README.md` (about, container usage, env vars, security notes)
+- [x] staticcheck 399 -> 0; dead code and duplicates removed; `ioutil`/`rand.Seed` gone. Real bugs fixed on the way: leaked file handle per ffmpeg segment, unchecked `http.NewRequest`, migration writing `null` xepg.json, silent user-write failures, unwritable config/temp dir now fatal at start
+- [ ] Resolve missing `docs/design-system/` referenced by `agent.md` (user decision: agent.md is a personal, git-ignored file)
 
 ## Phase 4: Data model and performance
 Not planned (lineup is ~170 channels, decided 2026-09-25). See plan for the reference list.