feat(config): resolve media URLs + URL timestamps in config-driven video (parity with quick playback)

[kfox]

Why

A video scene's file only accepted local paths or direct media URLs in config mode. A YouTube (or any yt-dlp) page URL was passed straight to PyAV, which failed at playback with ffmpeg's cryptic Invalid data found when processing input. yt-dlp resolution (and the ?t= start-offset parsing) only happened in the positional c64cast MEDIA… quick-playback path. This brings the two interfaces to parity.

What

[[scenes]]
type = "video"
file = "https://youtu.be/<id>?t=18m18s"   # resolved at load; starts at 18:18

• One shared resolver — quickcast.resolve_video_url (yt-dlp resolution +
  audio-only rejection + t=/start=/#t= timestamp). config.build_scene
  calls it for a single-URL video file, folding the URL timestamp into
  start_s (an explicit start_s still wins) and using the resolved title as
  the scene name. Local file / dir / glob / multi specs are untouched.
• Single resolution site — quickcast.classify_url no longer pre-resolves;
  it stores the URL verbatim and parses the timestamp offline, so resolution
  happens once, in build_scene, for both the positional path and configs.
• Better diagnostics — --doctor and config load now flag (offline) a
  single URL that needs yt-dlp when the yt extra is missing, with an install
  hint, instead of the runtime ffmpeg failure. Direct media URLs need no extra.

Docs

CLAUDE.md (quickcast + config sections), docs/usage.md (type = "video"), and
the example TOML document URL file specs + the timestamp behavior.

Tests

• resolve_video_url (video tuple / audio rejection), url_needs_ytdlp.
• build_scene URL resolution: stream URL + timestamp→start_s + title→name;
  explicit start_s wins; audio-only rejected. (yt-dlp + network faked.)
• Offline yt-extra doctor check: rejected without the extra, passes with it,
  direct URLs exempt.

Full suite green; schema unchanged (no new field).

[codecov]

Codecov Report

❌ Patch coverage is 84.84848% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.55%. Comparing base (e37cbee) to head (f79233b).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
c64cast/config.py	86.36%	1 Missing and 2 partials ⚠️
c64cast/quickcast.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #38   +/-   ##
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  Coverage   79.54%   79.55%           
=======================================
  Files          68       68           
  Lines       12817    12847   +30     
  Branches     1889     1896    +7     
=======================================
+ Hits        10195    10220   +25     
- Misses       2185     2188    +3     
- Partials      437      439    +2     

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