Library: poll clip readiness one pass at a time

[pallaoro]

Targets main and includes #44's commit (same hook). Merge in either order: if #44 merges first, its change is already identical here; if this merges first, #44 has nothing left and can be closed.

useMediaReady asked about each pending clip in turn and started a new pass every 5 s with setInterval, whether or not the previous pass had finished. A /playback answer takes ~1.7 s, so a pass over a few hundred clips runs for minutes. The passes piled up, and each one kept asking about every clip, including after the hook's set had changed: dead was only checked once the whole loop was done.

• src/shared/passes.ts: repeatPasses(pass, gapMs) runs a pass, waits gapMs after it finishes, then runs the next one. It never runs two at once. stop() cancels the next pass, and the pass under way sees stopped() and gives up partway.
• useMediaReady uses it, and checks stopped() before each clip.

Test (test/passes.test.ts): models 400 clips at 1.7 s each against a 5 s gap. Run against the old setInterval loop, the same test reaches 137 passes at once, and it fails 3 of its 4 cases.

vitest: 136 passed. footage and highlights fail to import @clawnify/queue in my local install, the same as on the base branch.

[pallaoro]

Superseded by #60, merged and deployed 2026-10-10: this PR's commit was cherry-picked onto today's main with its author kept (88f5567 → d5117be, 4bb67d5 → 22e6694), plus one addition: a clip that is gone (404, or 502 not_found from the media service) also stops being polled.