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>

src/downloader.rs

@@ -30,11 +30,12 @@ use std::process::{Command, Stdio};
 use std::sync::mpsc::{channel, Receiver};
 use std::thread;
 
+use crate::download_options::DownloadOptions;
 use crate::platform::{self, Platform, UrlInfo, UrlKind};
 use crate::ytdlp_bin;
 
 /// Video quality level passed as a `-f` format selector to yt-dlp.
-#[derive(Clone, Copy, PartialEq, Eq, Default)]
+#[derive(Clone, Copy, PartialEq, Eq, Default, serde::Serialize, serde::Deserialize, Debug)]
 pub enum DownloadQuality {
     /// No `-f` flag — yt-dlp picks the best available streams (default).
     #[default]
@@ -294,7 +295,22 @@ impl Downloader {
     /// added, `--break-on-existing` is suppressed (each recording is unique),
     /// and the output filename gains a UTC timestamp suffix so re-recordings
     /// of the same channel don't collide.
-    pub fn start(&mut self, url: String, info: &UrlInfo, full_scan: bool, quality: DownloadQuality, live: bool) {
+    ///
+    /// `channel_options` carries per-channel overrides (rate limit, filters,
+    /// extra args, …) and is applied after the standard flag set so it can
+    /// win. Pass `None` when the caller doesn't know which channel the URL
+    /// belongs to (e.g. an arbitrary URL pasted into the download dialog).
+    /// The channel-options `quality` field overrides the `quality` parameter
+    /// only when the caller explicitly opts in by passing it through.
+    pub fn start(
+        &mut self,
+        url: String,
+        info: &UrlInfo,
+        full_scan: bool,
+        quality: DownloadQuality,
+        live: bool,
+        channel_options: Option<&DownloadOptions>,
+    ) {
         let platform_dir = platform::platform_root(&self.channels_root, info.platform);
         // Per-platform download archive keeps cross-platform IDs from colliding
         // (TikTok IDs are numeric, YouTube IDs are 11-char base64, etc.).
@@ -391,6 +407,12 @@ impl Downloader {
             .arg(archive_path.display().to_string());
         self.apply_impersonation(info.platform, &mut cmd);
         self.apply_platform_extras(info.platform, &mut cmd);
+        // Per-channel option overrides win when present — they're applied
+        // last so a `--limit-rate` / `--match-filter` / passthrough arg from
+        // the channel settings takes priority over the global defaults.
+        if let Some(opts) = channel_options {
+            opts.apply(&mut cmd);
+        }
         cmd.arg("-o").arg(&out_arg).arg(&url);
         Self::apply_retry_flags(&mut cmd);