feat(web): per-video chat over the stored transcript (ADR-027)

A deeper-dive chat entered from the summary view: ask questions about a video
against its already-stored transcript (ADR-021), no caption fetch, ever.

Safety by construction — the load-bearing property. The chat handlers reach the
chat.Service only after reading the SHARED stored transcript via Store.GetTranscript
(a pure DB read); the service holds no VideoSource. So an enabled chat cannot
trigger a caption fetch, touch the rate gate, or reach YouTube. A video with no
stored transcript gets an honest "not available" — no fetch, no model call. The
key web test wires the summarize/fetch collaborators as tripwires that fail the
test if chat ever routes into them, and asserts the model answered from the
stored text.

Model defaults to the summary's own model and is switchable among the ADR-022
chain (phi4-mini → gemma4-26b → mistral-small); switching re-runs against the
same transcript — deliberate model-comparison instrumentation. The cloud model
is absent from the switcher when TAPIR_CLOUD_FALLBACK_MODEL="" (the local-first /
NDA lever), honoured the same way the summarizer honours it. Reuses the existing
LiteLLM gateway client (a chat is a different call, not a new integration) and
the TAPIR_MAX_TRANSCRIPT_CHARS truncation, surfacing an honest bounded-context
note when a long transcript is cut.

Ephemeral v1: the multi-turn conversation rides in hidden request fields; no
table, no migration, nothing persisted. Entry is RLS-scoped through
GetSummaryByVideo, so chat is reachable only from the user's own summary view.
Show-source verification and on-demand fetch are deferred (ADR-027).

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

internal/web/handlers.go

@@ -26,6 +26,10 @@ type Store interface {
 	DistinctChannels(ctx context.Context, userID string) ([]string, error)
 	GetSummaryByVideo(ctx context.Context, userID, videoID string) (*store.SummaryRow, error)
 	GetVideoRow(ctx context.Context, userID, videoID string) (*store.SummaryRow, error)
+	// GetTranscript reads the shared, stored transcript keyed by (provider,
+	// providerVideoID) — ADR-021. It is a pure DB read: it never fetches captions,
+	// so the chat path (ADR-027) reaches it without any caption-fetch surface.
+	GetTranscript(ctx context.Context, provider, providerVideoID string) (domain.Transcript, bool, error)
 	ActionsFor(ctx context.Context, userID string, videoIDs []string) (map[string][]string, error)
 	SetAction(ctx context.Context, userID, videoID, action string) error
 	ClearAction(ctx context.Context, userID, videoID, action string) error
@@ -91,6 +95,11 @@ type App struct {
 	// Fetcher, when non-nil, resolves an arbitrary YouTube video id to metadata for
 	// the paste-a-URL flow (Feature 2). Nil = the /paste route is not mounted.
 	Fetcher VideoFetcher
+	// Chat, when non-nil, answers per-video questions against a video's STORED
+	// transcript (ADR-027). Nil = the /v/{id}/chat routes are not mounted and the
+	// summary view shows no "dig deeper" affordance. It holds no caption-fetch
+	// dependency, so an enabled chat cannot reach YouTube or the rate gate.
+	Chat Chatter
 	// Processing tracks in-flight immediate summarizations so the status endpoint
 	// shows the animation until the summary lands. The zero value is ready to use.
 	Processing ProcessingSet
@@ -149,6 +158,12 @@ func (a *App) Router() http.Handler {
 		app.HandleFunc("POST /paste", a.handlePaste)
 	}
 	app.HandleFunc("GET /v/{videoId}/status", a.handleStatus)
+	// Per-video deeper-dive chat over the STORED transcript (ADR-027). Mounted only
+	// when a Chat backend is wired; it never fetches captions.
+	if a.Chat != nil {
+		app.HandleFunc("GET /v/{videoId}/chat", a.handleChat)
+		app.HandleFunc("POST /v/{videoId}/chat", a.handleChatMessage)
+	}
 	app.HandleFunc("GET /register", a.handleRegisterForm)
 	app.HandleFunc("POST /register", a.handleRegister)
 
@@ -265,7 +280,7 @@ func (a *App) handleDetail(w http.ResponseWriter, r *http.Request) {
 		a.serverError(w, r, "get summary", err)
 		return
 	}
-	a.render(w, r, DetailPage(*row))
+	a.render(w, r, DetailPage(*row, a.Chat != nil))
 }
 
 // handleAction toggles one action: re-clicking an active verb clears it, else it