Grilling: extension URL contract for watch-sessions

Part of #78

Blocked by: [Research: extension URL payload — the CDP contract replacement](https://github.com/adityanandanx/heimdall/issues/80)

## Question

With CDP removed, **what is the session-level contract for media URLs**?

- Which fields flow from the extension into the watch-session (url, video id, title, tab position?) and on which events (URL change while playing, tab switch, MPRIS play/pause)?
- How do we correlate an extension message with an open MPRIS session when several Chromium tabs/players are live (multiplayer stream)?
- What happens to `media_id`/`media_source` of already-captured rows, and to search/recap inputs, when the source changes (extension vs CDP)?

Deliverable: the URL-source contract, replacing `_enrich_chromium_media`/`cdp_resolve`, plus the migration note (what breaks, what FTS still sees).