feat(summarizer): resilient endpoint chain with local→cloud fallback (ADR-022)

The first friendly-pilot live run produced zero summaries: koala/phi4-mini hit
three silent failure modes — 8k context overflow on long transcripts (HTTP 400),
intermittent malformed JSON (highlights as a bare string), and no fallback wired
at all (summarizer.New(primary, nil)).

Keep phi4-mini as the fast primary and add resilience around it:

- Ordered endpoint chain (summarizer.NewChain): phi4-mini → koala/phi4-14b
  (local) → berget/mistral-small (worst-case external). All reached through the
  one LiteLLM gateway by alias.
- A parse failure now advances the chain like a transport error — the old
  Primary→Fallback shape returned the parse error without trying anyone else.
- Tolerant parse: highlights/takeaways coerce string→[]string, absorbing the
  common small-model quirk without spending a fallback round-trip.
- Transcript truncation (TAPIR_MAX_TRANSCRIPT_CHARS=18000) prevents the overflow
  rather than recovering from it; validated to fit phi4-mini's 8k window.
- Bounded completion budget (TAPIR_SUMMARY_MAX_TOKENS=1500) — the old 8192 budget
  itself contributed to the overflow.

Local-first guarantee preserved by ordering: external endpoint is tried only
after every local one fails. TAPIR_CLOUD_FALLBACK_MODEL="" disables it entirely
for client/NDA deployments.

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

internal/adapters/summarizer/summarizer_test.go

@@ -129,8 +129,8 @@ func TestSummarize_NoBYO_ContentOnlyLocal(t *testing.T) {
 	local := &fakeClient{reply: goodReply}
 	s := New(Endpoint{Client: local, Provider: "local", Model: "iguana/deepseek-r1-14b"}, nil)
 
-	if s.fallback != nil {
-		t.Fatal("no BYO configured but fallback endpoint is non-nil")
+	if len(s.endpoints) != 1 {
+		t.Fatalf("no BYO configured but chain has %d endpoints, want 1", len(s.endpoints))
 	}
 	for i := 0; i < 3; i++ {
 		sum, err := s.Summarize(context.Background(), testVideo(), testTranscript())
@@ -176,3 +176,97 @@ func TestParse_EmptySummaryRejected(t *testing.T) {
 		t.Fatal("want error for empty summary (thinking model returned no content)")
 	}
 }
+
+// parse tolerates a small model emitting "highlights" as a bare string instead
+// of an array — the production koala/phi4-mini quirk that errored with
+// "cannot unmarshal string into Go struct field ... highlights of type []string".
+func TestParse_ToleratesStringHighlights(t *testing.T) {
+	p, err := parse(`{"summary":"s","highlights":"one big point","takeaways":["a","b"]}`)
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if len(p.Highlights) != 1 || p.Highlights[0] != "one big point" {
+		t.Errorf("highlights = %v, want [\"one big point\"]", p.Highlights)
+	}
+	if len(p.Takeaways) != 2 {
+		t.Errorf("takeaways = %v, want 2", p.Takeaways)
+	}
+}
+
+// Chain: an endpoint that returns a 200 with unparseable output is treated as a
+// failure, and the next endpoint in the chain is tried. This is the case the old
+// primary->fallback shape missed — a parse error short-circuited instead of
+// falling back.
+func TestSummarize_FallsBackOnMalformedOutput(t *testing.T) {
+	bad := &fakeClient{reply: `{"summary": not json`}
+	good := &fakeClient{reply: goodReply}
+	s := NewChain([]Endpoint{
+		{Client: bad, Provider: "local", Model: "koala/phi4-mini"},
+		{Client: good, Provider: "local", Model: "koala/phi4-14b"},
+	}, 0)
+
+	sum, err := s.Summarize(context.Background(), testVideo(), testTranscript())
+	if err != nil {
+		t.Fatalf("Summarize: %v", err)
+	}
+	if sum.AIModel != "koala/phi4-14b" {
+		t.Errorf("AIModel = %q, want koala/phi4-14b (fell back past malformed primary)", sum.AIModel)
+	}
+	if !sum.FallbackUsed {
+		t.Error("FallbackUsed = false, want true")
+	}
+	if bad.calls != 1 || good.calls != 1 {
+		t.Errorf("calls: bad=%d good=%d, want 1 and 1", bad.calls, good.calls)
+	}
+}
+
+// Chain: when every endpoint fails, no summary is produced and the joined error
+// names each failure so the engine queues the work for retry.
+func TestSummarize_ChainAllEndpointsFail(t *testing.T) {
+	a := &fakeClient{err: errors.New("context overflow")}
+	b := &fakeClient{reply: "not even json"}
+	s := NewChain([]Endpoint{
+		{Client: a, Provider: "local", Model: "m1"},
+		{Client: b, Provider: "berget", Model: "m2"},
+	}, 0)
+
+	if _, err := s.Summarize(context.Background(), testVideo(), testTranscript()); err == nil {
+		t.Fatal("want error when all endpoints fail")
+	}
+	if a.calls != 1 || b.calls != 1 {
+		t.Errorf("calls: a=%d b=%d, want 1 and 1", a.calls, b.calls)
+	}
+}
+
+// A transcript longer than the chain's input budget is truncated before it
+// reaches any model, so a small-context primary does not overflow its window.
+func TestSummarize_TruncatesLongTranscript(t *testing.T) {
+	local := &fakeClient{reply: goodReply}
+	const budget = 100
+	s := NewChain([]Endpoint{{Client: local, Provider: "local", Model: "m"}}, budget)
+
+	long := domain.Transcript{
+		VideoID: "vid-1", UserID: "user-1", Source: domain.SourceCaptions,
+		Content: strings.Repeat("word ", 1000), // 5000 bytes, well over budget
+	}
+	if _, err := s.Summarize(context.Background(), testVideo(), long); err != nil {
+		t.Fatalf("Summarize: %v", err)
+	}
+	// The prompt carries title/URL framing plus the truncation marker, so allow
+	// headroom over the raw transcript budget — but it must be far below 5000.
+	if len(local.lastUser) > budget+300 {
+		t.Errorf("prompt length = %d, want <= %d (transcript not truncated)", len(local.lastUser), budget+300)
+	}
+	if !strings.Contains(local.lastUser, "truncated") {
+		t.Error("truncation marker missing from prompt")
+	}
+}
+
+func TestNewChain_PanicsOnEmptyChain(t *testing.T) {
+	defer func() {
+		if recover() == nil {
+			t.Fatal("want panic on empty endpoint chain")
+		}
+	}()
+	NewChain(nil, 0)
+}