fix: harden download lifecycle, subscriptions, and UI robustness

Addresses a full-project review. Backend correctness and availability:

- ytdl: cancel() only SIGKILLs the child's process group when the child
  actually became its own group leader, so a race (or failed setpgrp)
  can no longer kill the whole server; kill the group on cancel and on
  shutdown to avoid orphaned ffmpeg children
- ytdl: dedicated ThreadPoolExecutor for download supervision so active
  downloads can't starve extract_info / live probes on the default pool
- ytdl/main/subscriptions: route fire-and-forget tasks through a
  bg_tasks helper that keeps a strong ref and logs failures
- subscriptions: run flat-playlist extraction in an executor and check
  feeds with bounded concurrency so one slow feed can't block the loop;
  set last_checked on failure so broken feeds aren't retried every 60s
- main: validate ids on /start & /delete and numeric env vars at startup;
  return 400 (not 500) on bad subscriptions/update input; serve /history
  from memory; move get_custom_dirs off the event loop; restrict t=
  stripping to YouTube hosts; drop double percent-decode in state guard
- dl_formats/ytdl: enforce requested caption format via
  FFmpegSubtitlesConvertor and strip VTT header metadata only in the
  pre-cue region so real dialogue is preserved
- ytdl: throttle progress events, dedup adds against pending, clear
  filename/size on error and reject out-of-dir trashcan deletes, pin
  fork start-method on Linux only

Frontend:

- retry deletes the done record only after a successful re-add
- surface HTTP errors for delete/start and reset the deleting flag
- ignore late 'updated' events for rows no longer in the queue
- track table rows by map key; FileSizePipe uses base-1024

Also: HTTPS-aware Docker healthcheck, dead-code removal, and shared
helpers for path-containment and yt-dlp option merging. Adds/updates
unit tests throughout (250 backend tests passing).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Dockerfile

@@ -1,4 +1,8 @@
-FROM node:lts-alpine AS builder
+# Pinned to a major version rather than the lts-alpine floating tag: that tag
+# has lagged behind and resolved to a Node patch older than the Angular CLI's
+# minimum supported version, breaking the build. node:22-alpine currently
+# satisfies @angular/cli's >=22.22.3 requirement.
+FROM node:22-alpine AS builder
 
 WORKDIR /metube
 COPY ui ./
@@ -66,7 +70,8 @@ ENV TEMP_DIR=/downloads
 ENV PORT=8081
 VOLUME /downloads
 EXPOSE 8081
-HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 CMD curl -fsS "http://localhost:${PORT}/" || exit 1
+HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 \
+  CMD case "$HTTPS" in true|True|on|1) curl -fsSk "https://localhost:${PORT}/";; *) curl -fsS "http://localhost:${PORT}/";; esac || exit 1
 
 # Add build-time argument for version
 ARG VERSION=dev