feat(gate): foreground caption fetches take priority over the background sweep (ADR-026)

A user waiting on a Summarize click shared the per-IP caption gate equally with
the background firehose, so on a busy IP the click was slow or 429'd. Add a
context-marked priority lane: the web path (engineProcessor.ProcessVideo) marks
its context foreground; the gate serves foreground immediately while background
fetches yield until no foreground is pending. Threaded via a context value (no
new signatures) + a process-wide foregroundPending counter. Clicks are rare, so
the background barely loses throughput; the waiting human gets the cleaner slot.

Drops the credentials probe: ADR-010 and captions.go already settle it — the
timedtext/InnerTube path rejects authenticated requests and the OAuth token does
not authenticate it anyway, so auth cannot help and can hurt. Documented in
ADR-026 rather than built.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

DECISIONS.md

@@ -958,6 +958,36 @@ schema change (reuses `transcript_status` from migration 007).
 
 ---
 
+## ADR-026 — Foreground caption fetches take priority; the credentials probe is dead
+
+**Status:** Accepted (2026-06-10). **Pillar A of the manual-mode UX work** (Pillar B was
+ADR-025). Builds on ADR-014 (the shared per-IP gate).
+
+**Context.** Every caption fetch — the background sweep and the web click-path — shared one
+process-wide rate gate equally. So a user waiting on a "Summarize" click competed with the
+firehose for both pacing and the scarce pre-429 window; on a busy IP the click was slow or
+429'd while the background churned.
+
+**Decision.** A context-marked priority lane. The web path
+(`engineProcessor.ProcessVideo`) wraps its context with `ForegroundContext`; the gate gives
+foreground fetches a token immediately, while **background fetches yield** — they wait until no
+foreground fetch is pending before taking a token. Threaded via a context value (not new
+signatures) and a process-wide `foregroundPending` counter. Clicks are rare and bursty, so the
+background barely loses throughput; the waiting human gets the next (and cleanest) slot.
+
+**Credentials probe — rejected, not built.** The idea was to fetch captions with the user's
+auth in manual mode to dodge 429s. It is a dead end, already settled by ADR-010 and the code:
+the caption path is *deliberately anonymous* because the InnerTube/timedtext endpoints **reject
+or break on authenticated requests** (`captions.go`: "no OAuth token — it can break the
+timedtext endpoint"). The user's OAuth (a Data API credential) does not authenticate InnerTube
+at all, and the official `captions.download` is owner-only (403 on third-party). So auth cannot
+help here and can actively hurt. No probe needed — building one would only re-confirm the ADR.
+
+**Reversibility.** Context-marker + a yield loop in the gate; removing the marker collapses to
+the prior equal-share behaviour. No schema or API change.
+
+---
+
 ## Rejected alternatives
 
 Approaches considered during the 2026-06-02 planning + grill session and **deliberately not