feat(chat): stored-transcript QA service (ADR-027)

A read-only deeper-dive over a video's already-stored transcript (ADR-021).
Safe by construction: the Service has no VideoSource and no caption-fetch
dependency — only a Completer factory over the existing LiteLLM gateway — so it
cannot reach YouTube or the rate gate. Reuses the summarizer's truncation
discipline (TAPIR_MAX_TRANSCRIPT_CHARS), reporting the cut so the UI can be
honest about a bounded transcript. Model defaults to the summary's model and is
switchable among an offered, local-first list; an un-offered alias is forced
back to the default so chat can never call the gateway with an arbitrary model.
Ephemeral: history is carried per-request, nothing persisted.

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

internal/adapters/chat/chat.go

@@ -0,0 +1,171 @@
+// Package chat implements the per-video deeper-dive chat (ADR-027): a read-only
+// QA over a video's ALREADY-STORED transcript (ADR-021). It is the enforcement
+// point for the feature's load-bearing safety property — stored-transcript-only:
+// the Service has NO VideoSource and NO caption-fetch dependency, only a
+// Completer factory, so it CANNOT reach YouTube or the rate gate by construction.
+// The caller supplies the stored transcript text; chat never fetches.
+//
+// It reuses the same LiteLLM gateway as the summarizer (a chat is a different
+// call, not a new integration) and the same transcript-truncation discipline
+// (TAPIR_MAX_TRANSCRIPT_CHARS) so a long transcript fits a small-context model.
+package chat
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"unicode/utf8"
+)
+
+// Completer is the minimal LLM chat surface the Service needs. *llm.Client
+// satisfies it; tests use a fake. It is the SAME surface the summarizer uses.
+type Completer interface {
+	Complete(ctx context.Context, system, user string) (string, error)
+}
+
+// Turn is one completed exchange in an ephemeral, session-only conversation
+// (ADR-027 v1: nothing is persisted).
+type Turn struct {
+	Question string
+	Answer   string
+}
+
+// Request is one chat turn: the chosen model, the stored transcript text, the
+// prior turns (for multi-turn context within the session), and the new question.
+type Request struct {
+	Model      string
+	Transcript string
+	History    []Turn
+	Question   string
+}
+
+// Reply is the model's answer plus whether the transcript was bounded to fit the
+// model context (so the UI can be honest that an answer about the tail of a long
+// video may be incomplete).
+type Reply struct {
+	Answer    string
+	Truncated bool
+}
+
+// Service answers questions against a stored transcript via a switchable set of
+// models. models is the ordered, local-first list offered to the user (the cloud
+// model is simply absent when disabled — see cmd wiring); maxChars bounds the
+// transcript sent to any model (0 = unbounded). newClient builds a Completer for
+// a chosen model alias (the same gateway, a different alias).
+type Service struct {
+	newClient func(model string) Completer
+	models    []string
+	maxChars  int
+}
+
+// New constructs a Service. models must be non-empty and already filtered to the
+// offerable set (cloud excluded when disabled) and de-duplicated by the caller.
+func New(newClient func(model string) Completer, models []string, maxChars int) *Service {
+	return &Service{newClient: newClient, models: models, maxChars: maxChars}
+}
+
+// Models returns a copy of the offerable model list (local-first order).
+func (s *Service) Models() []string {
+	out := make([]string, len(s.models))
+	copy(out, s.models)
+	return out
+}
+
+// offers reports whether model is in the offerable set — the guard that keeps an
+// arbitrary, un-offered alias (e.g. a forged form value) from reaching the gateway.
+func (s *Service) offers(model string) bool {
+	for _, m := range s.models {
+		if m == model {
+			return true
+		}
+	}
+	return false
+}
+
+// DefaultModel resolves the model a fresh chat opens with: the summary's own
+// model when it is still an offered option (the ADR-027 default — chat continues
+// in the model that produced the summary), otherwise the first offered model.
+// Returns "" only when no models are configured.
+func (s *Service) DefaultModel(summaryModel string) string {
+	if summaryModel != "" && s.offers(summaryModel) {
+		return summaryModel
+	}
+	if len(s.models) > 0 {
+		return s.models[0]
+	}
+	return ""
+}
+
+// Answer runs one chat turn. The model is forced back to a default if the request
+// names an un-offered alias, so chat can never call the gateway with an arbitrary
+// model. The transcript is truncated up front (reporting whether it was cut) and
+// passed as system context; the running conversation is the user message.
+func (s *Service) Answer(ctx context.Context, req Request) (Reply, error) {
+	if len(s.models) == 0 {
+		return Reply{}, fmt.Errorf("chat: no models configured")
+	}
+	model := req.Model
+	if !s.offers(model) {
+		model = s.DefaultModel("")
+	}
+
+	transcript, truncated := truncate(req.Transcript, s.maxChars)
+	system := buildSystem(transcript, truncated)
+	user := buildUser(req.History, req.Question)
+
+	out, err := s.newClient(model).Complete(ctx, system, user)
+	if err != nil {
+		return Reply{}, fmt.Errorf("chat: %s: %w", model, err)
+	}
+	answer := strings.TrimSpace(out)
+	if answer == "" {
+		return Reply{}, fmt.Errorf("chat: %s returned an empty answer", model)
+	}
+	return Reply{Answer: answer, Truncated: truncated}, nil
+}
+
+const systemPreamble = `You are Tapir, answering questions about ONE video using ONLY the transcript below.
+Ground every answer in the transcript. If the transcript does not contain the answer, say so plainly rather than guessing.`
+
+const truncatedNote = `
+The transcript below is truncated to fit the model — if a question seems to concern something missing, note it may be beyond the available portion.`
+
+// buildSystem frames the model as a transcript-grounded QA assistant and embeds
+// the (possibly truncated) transcript as context.
+func buildSystem(transcript string, truncated bool) string {
+	var b strings.Builder
+	b.WriteString(systemPreamble)
+	if truncated {
+		b.WriteString(truncatedNote)
+	}
+	b.WriteString("\n\nTranscript:\n")
+	b.WriteString(transcript)
+	return b.String()
+}
+
+// buildUser renders the running conversation as the user message: prior turns as
+// Q/A pairs followed by the new question. Folding history into one message keeps
+// the Completer surface (a single system+user call) unchanged — no new llm method.
+func buildUser(history []Turn, question string) string {
+	var b strings.Builder
+	for _, t := range history {
+		fmt.Fprintf(&b, "Q: %s\nA: %s\n\n", t.Question, t.Answer)
+	}
+	fmt.Fprintf(&b, "Q: %s", question)
+	return b.String()
+}
+
+// truncate caps content to max bytes on a UTF-8 rune boundary, reporting whether
+// it cut. It mirrors the summarizer's truncation discipline (ADR-022) but returns
+// the cut flag so the chat UI can be honest about a bounded transcript. A
+// non-positive max (or content already within budget) returns content unchanged.
+func truncate(content string, max int) (string, bool) {
+	if max <= 0 || len(content) <= max {
+		return content, false
+	}
+	cut := max
+	for cut > 0 && !utf8.RuneStart(content[cut]) {
+		cut--
+	}
+	return content[:cut], true
+}