Per-channel download options (Tartube parity Phase 1.1)

Closes the single largest gap with Tartube documented in docs/tartube-spec.md.
Users can now attach a small set of yt-dlp overrides to any individual
channel; the overrides apply automatically during scheduled re-checks
and the right-click "Check for new videos" action.

Data layer
- New `channel_options` SQLite table keyed on (platform, handle), with
  `options_json TEXT` blob + updated_at. PK ensures one row per channel.
- Database gains get_channel_options / set_channel_options /
  delete_channel_options / get_all_channel_options. The bulk getter is
  used by the library scanner so each rescan hydrates options without
  per-channel SQL round-trips.
- `DownloadQuality` derives Serialize/Deserialize so it can roundtrip
  through the options blob.

New module: src/download_options.rs
- `DownloadOptions` struct with 9 fields:
    quality (Option<DownloadQuality>) — per-channel quality cap
    audio_only (bool) — force --extract-audio chain
    limit_rate_kb (Option<u32>) — --limit-rate
    min_filesize_mb / max_filesize_mb — yt-dlp filesize filters
    date_after (Option<String>) — --dateafter YYYYMMDD
    match_filter (Option<String>) — yt-dlp --match-filter passthrough
    subtitle_langs (Vec<String>) — --sub-langs CSV
    extra_args (Vec<String>) — raw yt-dlp passthrough (Tartube's
        `extra_cmd_string`)
- `apply(&mut Command)` appends the right flags conditionally.
- `is_empty()` lets the API layer collapse no-op writes into DELETEs so
  we don't keep useless rows around.
- `from_json()` falls back to defaults on malformed rows so a schema
  drift doesn't take a channel offline.
- 8 new unit tests cover apply-emits-correct-flags, JSON roundtrip,
  corrupt-fallback, and is_empty.

Library
- `Channel` gains `download_options: DownloadOptions` (default empty).
- New `library::apply_channel_options(channels, map)` helper that the
  caller invokes after `scan_channels` to hydrate from the bulk DB load.
- All five `scan_channels` call sites updated.

Downloader
- `Downloader::start` gains a new `channel_options: Option<&DownloadOptions>`
  parameter. Applied last so per-channel overrides win over global
  defaults. The explicit "submit from URL bar" flow passes None
  (channel unknown until yt-dlp resolves it). Scheduled re-checks +
  right-click checks pass the channel's stored options. Channel-options'
  `quality` field overrides the hard-coded DownloadQuality::Best for
  re-checks.

Web API
- New routes on /api/channels/:platform/:handle/options:
    GET    — fetch (returns defaults when no row exists)
    POST   — upsert (empty body collapses to DELETE)
    DELETE — clear overrides
- All three update the in-memory library snapshot immediately so the
  next re-check sees the change without waiting for a rescan, and bump
  the library_version ETag so polling clients pick up the new state.

Web UI
- Sidebar each channel's expanded section gains a "⚙ Channel options…"
  entry next to "Check for new videos".
- openChannelOptions() fetches the current options + renders a modal
  with one input per field. Save / Clear / Cancel buttons. The Clear
  button confirms via window.confirm() because it's destructive.
- All form fields validate locally before POSTing; empty numeric
  fields map to null.

Desktop UI
- New `channel_options_window` egui::Window opened from the existing
  channel right-click context menu (new "⚙ Channel options…" item).
- Form mirrors the web side: ComboBox for quality, checkbox for
  audio-only, TextEdit (singleline + multiline) for the other fields.
- Save / Clear / Cancel buttons; saves go straight to SQLite and the
  live `App::library` so the next scheduled-check or right-click runs
  with the new overrides.

Documentation
- `docs/tartube-spec.md` checklist: per-target download options ticked
  with a note describing the v1 scope.

55 unit tests pass (47 + 8 new).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/tartube-spec.md

@@ -571,8 +571,13 @@ For the record (to keep momentum honest):
 Working definition: a yt-offline release is "at Tartube parity" when
 the following are all true.
 
-- [ ] **Per-target download options** with cascade resolution and a
-      per-channel editor.
+- [x] **Per-target download options** with cascade resolution and a
+      per-channel editor. *v1 shipped 2026-05-25: 9 fields per channel
+      (quality cap, audio-only, rate limit, min/max filesize, date-after,
+      match-filter, subtitle langs, raw extra args), persisted in the
+      `channel_options` SQLite table, attached at scan time, applied at
+      every dispatch site (scheduled re-checks, right-click checks).
+      Editor surfaces in both UIs.*
 - [ ] **Folder hierarchy** with N-level nesting, drag-to-reparent, and
       per-folder default options.
 - [ ] **Smart folders** for Bookmarks / Favourites / Waiting / New /