docs(decisions): add ADR-027 — chat with a video's stored transcript

Records the deeper-dive chat decision before its build: stored-transcript-only
(safe by construction — no caption fetch, no rate gate, no YouTube), entered
from the summary view, model defaulting to the summary's model and switchable
among the ADR-022 chain. Ephemeral v1; show-source verification deferred to v2.
Inserted before "Rejected alternatives" so the decision precedes the feature.

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

DECISIONS.md

@@ -996,6 +996,70 @@ the prior equal-share behaviour. No schema or API change.
 
 ---
 
+## ADR-027 — Chat with a video's stored transcript (deeper-dive, on an already-summarized video)
+
+**Status:** Accepted (2026-06-11). **Consumes ADR-021** (the shared, video-keyed transcript
+store) for the first time beyond summarization; **uses the ADR-022 chain models**; relates to
+ADR-012 (isolation) and ADR-016 (the Stage-0 gate).
+
+**Context — observed demand, not hypothetical.** The maintainer read 10+ real pilot summaries
+and reported the reactions: *many good; some he wanted to dig deeper into; some less useful*
+(the "less useful" split between weak-*model* output and uninteresting-*video* content). The
+middle reaction is the signal: a good summary that makes the reader want *more* is the summary
+succeeding at triage and then hitting a wall — there is nowhere to go deeper short of watching
+the video. That want is the feature. It is also the cheapest possible feature to satisfy
+honestly, because ADR-021 already persists the transcript: the deeper-dive runs entirely on
+stored public-content text + local models, touching **no** caption fetch and **no** YouTube.
+
+**Decision.** Add a per-video chat that lets the user ask questions against a video's stored
+transcript.
+1. **Entry from the summary view only.** A "dig deeper / ask" affordance on a summarized video —
+   the chat lives exactly where the "I want more" reaction happens. No standalone chat surface.
+2. **Stored-transcript-only (load-bearing constraint).** Chat is available **only** for videos
+   that already have a stored transcript. It never triggers a caption fetch, so it cannot touch
+   the rate gate, the 429 surface, or YouTube at all — the entire account-safety constraint that
+   governs the rest of Tapir is satisfied *by construction* here, not by careful gating. (Entry
+   being "from a summary" guarantees the transcript exists.) On-demand fetch for un-stored videos
+   is explicitly deferred.
+3. **Model = the summary's model by default; user-switchable among the ADR-022 chain models**
+   (`phi4-mini` / `gemma4-26b` / `mistral-small` to start). This is deliberate: it doubles as
+   live model-comparison instrumentation — ask the same question of the same transcript under two
+   models and the difference is directly felt. This is the mechanism by which the maintainer
+   learns *which* model is worth defaulting to, and it is the multi-model-analysis direction
+   ADR-021 anticipated, arriving as a user-facing capability.
+   - Chat is a **read-bounded retrieval/QA task** (the user supplies the focus), which is
+     *easier* than summarization (the model must decide what matters). So a model that summarizes
+     mediocrely may chat well — chat is plausibly a partial remedy for the weak-summary case, not
+     an inheritor of it.
+4. **Ephemeral chat (v1).** No persisted history; chat is per-session. Persisting per-user,
+   RLS-scoped history is deferred until there is evidence anyone wants to revisit a conversation.
+5. **Chat-only, trust-the-model (v1) — with a recorded limitation.** The chat does not expose the
+   raw transcript for verification in v1 (kept simple). **Known limitation:** because some
+   summaries were weak-model output, the user has reason not to fully trust a chat answer's
+   fidelity to the transcript, and v1 gives no in-UI way to check. The model-switcher partially
+   compensates (two models disagreeing on the same question is itself a signal). A
+   "show source / view transcript" verification path is the natural **v2** and is *not*
+   foreclosed — ADR-021's stored transcript already makes it cheap. Recorded so v2 is a known
+   next step, not a rediscovery.
+
+**Why this is safe and in-scope.** It adds no caption-fetch surface (stored-only), no new
+non-RLS table (transcripts already shared per ADR-021; ephemeral chat stores nothing), and no
+auth change. It is additive to the read path. The one genuine product expansion — Tapir becomes
+an interactive transcript-QA tool, not only a summarizer — is justified by *observed* demand from
+real reading, which is exactly the kind of evidence the Stage-0 discipline asks for before
+building.
+
+**Relation to the Stage-0 gate.** This is **not** a return-nudge (those stay deferred, ADR-020) —
+it adds nothing that prompts the user to return; it deepens the value *once they are already
+reading*. It does not contaminate the unprompted-return signal. If anything it strengthens the
+"useful to me" case the gate measures, by giving a good summary somewhere to lead.
+
+**Reversibility.** Additive read-path feature over the unchanged engine + the ADR-021 store.
+Removing the summary-view affordance removes the feature; nothing else depends on it. Ephemeral =
+no migration, no stored state to unwind. Spec: `docs/specs/chat-with-transcript.md`.
+
+---
+
 ## Rejected alternatives
 
 Approaches considered during the 2026-06-02 planning + grill session and **deliberately not