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>

cmd/tapir/processor.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"fmt"
+	"strings"
 
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/llm"
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/secrets"
@@ -42,6 +43,40 @@ func (f videoFetcher) FetchVideo(ctx context.Context, userID, videoID string) (d
 // queue-only fallback: the web UI keeps working (the button just queues) and
 // `tapir run` reports the gap via its own ValidateForRun. Missing engine config
 // is never an error here.
+// buildSummarizer wires the summarization endpoint chain (ADR-022) shared by the
+// web "Summarize now" path and the scheduler's per-user runners. The chain is:
+// primary (local, fast) → local fallback → cloud fallback (worst case). Each
+// endpoint reaches the same LiteLLM gateway with a different model alias — the
+// gateway fronts both llama-swap and berget — so a fallback is just a different
+// alias, not a second client config. Empty model entries are skipped, so a
+// client deployment can set the cloud fallback empty to keep content local.
+func buildSummarizer(cfg config.Config) *summarizer.Summarizer {
+	mk := func(model string) summarizer.Endpoint {
+		return summarizer.Endpoint{
+			Client:   llm.New(cfg.GatewayURL, cfg.GatewayKey, model, cfg.SummarizerTimeout, llm.WithMaxTokens(cfg.SummaryMaxTokens)),
+			Provider: providerOf(model),
+			Model:    model,
+		}
+	}
+	eps := []summarizer.Endpoint{mk(cfg.SummarizerModel)}
+	if cfg.FallbackModel != "" && cfg.FallbackModel != cfg.SummarizerModel {
+		eps = append(eps, mk(cfg.FallbackModel))
+	}
+	if cfg.CloudFallbackModel != "" && cfg.CloudFallbackModel != cfg.SummarizerModel {
+		eps = append(eps, mk(cfg.CloudFallbackModel))
+	}
+	return summarizer.NewChain(eps, cfg.MaxTranscriptChars)
+}
+
+// providerOf maps a model alias to the domain AIProvider recorded on summaries.
+// A "berget/" alias is an external provider; everything else is the local stack.
+func providerOf(model string) string {
+	if strings.HasPrefix(model, "berget/") {
+		return "berget"
+	}
+	return "local"
+}
+
 func buildProcessor(cfg config.Config, st *store.Store) (*usecase.Engine, error) {
 	if cfg.GatewayURL == "" || cfg.YTClientID == "" || cfg.YTClientSecret == "" || cfg.SecretsFile == "" {
 		return nil, nil
@@ -55,13 +90,7 @@ func buildProcessor(cfg config.Config, st *store.Store) (*usecase.Engine, error)
 		PreferredLanguages: []string{"en"},
 	}, secretStore)
 
-	// Local Primary only; no BYO fallback for the demo (fallback nil).
-	primary := summarizer.Endpoint{
-		Client:   llm.New(cfg.GatewayURL, cfg.GatewayKey, cfg.SummarizerModel, cfg.SummarizerTimeout),
-		Provider: "local",
-		Model:    cfg.SummarizerModel,
-	}
-	sum := summarizer.New(primary, nil)
+	sum := buildSummarizer(cfg)
 
 	// The store is both the summary sink and the shared transcript cache (ADR-021):
 	// the engine reads stored transcripts before any caption fetch and writes

cmd/tapir/scheduler.go

@@ -6,9 +6,7 @@ import (
 	"log/slog"
 	"time"
 
-	"gitea.d-ma.be/mathias/tapir/internal/adapters/llm"
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/store"
-	"gitea.d-ma.be/mathias/tapir/internal/adapters/summarizer"
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/youtube"
 	"gitea.d-ma.be/mathias/tapir/internal/config"
 	"gitea.d-ma.be/mathias/tapir/internal/ports"
@@ -37,12 +35,7 @@ func buildUserRunner(cfg config.Config, st *store.Store, secretStore ports.Secre
 		PreferredLanguages: []string{"en"},
 	}, secretStore)
 
-	primary := summarizer.Endpoint{
-		Client:   llm.New(cfg.GatewayURL, cfg.GatewayKey, cfg.SummarizerModel, cfg.SummarizerTimeout),
-		Provider: "local",
-		Model:    cfg.SummarizerModel,
-	}
-	engine := usecase.NewEngine(src, summarizer.New(primary, nil), st)
+	engine := usecase.NewEngine(src, buildSummarizer(cfg), st)
 
 	return runner.New(src, st, engine, userID, log,
 		runner.WithBackoff(cfg.FetchBackoff),