fix(gateway): serialize STT/TTS reliability slice behind V6 contract

[andrexibiza]

Current disposition — candidate_blocked (Discord V6)

Canonical campaign authority: #90321, validated by the generic ledger contract in #90307.

Exact current head: 8e5cf6e57ca547da5a7b79a445cf5b66c426d9e0.

This branch contains real STT/TTS reliability fixes, but it is not the complete V6 publication authority by itself. V6 remains a paired/collision gap: open #78180 changes adjacent Discord voice config, persistence, command, gateway/run.py, and adapter surfaces. The campaign needs an explicit addendum and serialized composition rather than merging two locally coherent voice trains and discovering the contract afterward.

What this branch proves

1. Bounded stalled transcription

Salvaged from #74051, authored by @mglavinic86:

• wraps the complete configured-provider plus local-fallback sequence in a gateway-level deadline;
• defaults stt_timeout_seconds to 45 seconds;
• fails open with an audio-path marker instead of pinning the chat lock forever.

2. Media-producer recovery and delivery visibility

Residual work from #65745, authored by @dhansxd:

• recovers producer identity from explicit tool-result tool_name / name fields when the paired assistant tool_calls row is absent;
• restores automatic delivery eligibility for reconstructed/persisted TTS/audio results;
• logs unsuccessful voice-reply SendResult values instead of silently discarding the failure.

Historical focused verification reported 61 passing tests on Windows plus clean diff/footgun checks. That evidence remains useful for this head; it is not exact-current-main acceptance.

V6 collision / contract gap

#78180 independently introduces per-chat transcribe-only mode, transcript destinations, persisted voice preferences, slash-command behavior, and additional STT drop-point logging. It touches the same broad voice-control surfaces while representing different product semantics.

Before either train is promoted as V6 completion, the paired addendum must decide:

• which behaviors are one capability versus separately reviewable slices;
• config and persistence ownership;
• handler/command sequencing and compatibility;
• attribution and merge order;
• exact negative behavior for stalled STT, passive transcription, file-only transcripts, media delivery, and disabled agent turns.

Required before merge

1. Publish the V6 paired addendum that serializes fix(gateway): serialize STT/TTS reliability slice behind V6 contract #78196 and feat(gateway): Discord VC transcribe-only mode and transcript file destination #78180 without collapsing distinct behaviors into one unreviewable train.
2. Preserve @mglavinic86 and @dhansxd provenance on the salvaged fixes.
3. Rebase the selected slice onto current main and resolve overlapping voice config/handler/persistence ownership deliberately.
4. Run exact-head STT timeout, transcript-mode, TTS/media routing, busy-session, voice-command, persistence, and cross-platform tests.
5. Reconcile the authoritative publication state in docs(discord): install canonical 42-row parity ledger #90321 only after the composed contract is real.

Historical relationships

• fix(gateway): bound voice transcription stalls #74051 — bounded voice transcription stalls.
• fix(gateway): handle silent audio and media producers #65745 — silent-audio/media-producer residuals.
• BUG: TTS/MEDIA voice messages never delivered via gateway (false-positive history dedup) #74928 — verified fixed on current main by prior work; no code carried here for that issue.
• docs(discord): six documentation drifts vs actual behavior + /voice join missing from slash UI #11349 D2 — verified absent; no documentation drift introduced.
• Part of [meta] Vox Lockin — eliminate the STT/TTS/voice bug class (10 lanes, 38 issues) #78207.
• Part of WhatsApp Feature Parity & Alignment Campaign — meta-issue #79890.

This PR is implementation evidence pending V6 composition. It is ready for review, but not merge-authorized until the paired V6 contract above is satisfied.

[andrexibiza]

V6 paired-contract interlock published in #91913

The authority-continuity manifest records this PR together with #78180 and deliberately assigns no independent V6 delivery owner yet.

This lane owns bounded transcription settlement, fail-open audio preservation, media-producer recovery, and delivery visibility inside the shared voice-turn state machine. It must consume the same agent-turn admission decision as #78180; timeout/fallback and media recovery cannot manufacture or bypass an agent turn.

Contract PR: #91913
Exact contract head: 170a3a0e67034abd7d6a2c69a16c292b4781720f.