feat: recursive media discovery in --extract-scan

[xAlcahest]

Summary

The scanner in scan.c only looked at the landing page, so sites that load the player one or two clicks away (episode page -> /watch?file= -> the real .mp4) produced an empty skeleton. Now when the landing page has no direct media, scan_page follows internal watch/play/embed links up to --extract-scan-depth hops (default 2, max 3), re-scans each body with the same scoring, and records the capture chain that reached the media so scan_emit_config can write a reusable multi-step get/var config. Series index pages are detected and emit a list plus a per-episode pipeline. The crawl is bounded (visited-set, SCAN_MAX_FETCHES, 8 MiB page cap) and the ad-host blocklist still applies.

Config generation no longer writes into the active dir: run_extract_scan prints the config, stashes a pending copy under .pending/<id>.conf keyed by a stable id derived from the source URL, and suggests flux --save-config <id> to install it active (refusing to clobber). Both the argv id and the config name parsed back out of the pending file are sanitized before they touch a path.

Changes

• src/scan.h: recursion caps (SCAN_MAX_FOLLOW, SCAN_MAX_CHAIN, SCAN_MAX_DEPTH, SCAN_MAX_FETCHES), a per-candidate capture chain, series fields, and a max_depth argument on scan_page.
• src/scan.c: link and episode heuristic tables, bounded recursive follow with a visited-set, chain provenance, series detection, and multi-step/series config emission; the direct-media path is unchanged.
• src/text.c: --extract-scan-depth and --save-config flags, the pending-store and stable-id workflow, path sanitization on save, and depth clamping with a notice.
• src/test_scan.c: new one-hop, two-hop, and series cases that roundtrip the generated config through extractor_parse/run/list, plus the existing cases adapted to the new scan_page signature.
• .github/workflows/ci.yml: scan e2e now covers the save-config flow, a path-traversal attempt, and depth validation.
• README.md, NEWS, ChangeLog: document the recursive scan, the depth flag, and the save-by-id workflow.