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>

test/acceptance/scenario_coverage_test.go

@@ -26,10 +26,11 @@ import (
 // longer matches a real non-pending scenario.
 var scenarioCoverage = map[string]string{
 	// ai_routing.feature
-	"Local AI produces the summary":                             "TestSummarize_LocalSucceeds",
-	"Local AI fails and the user has a BYO provider configured": "TestSummarize_FallsBackToBYO",
-	"Local AI fails and the user has no BYO provider":           "TestSummarize_LocalFailsNoBYO_NoExternalSend",
-	"A user without BYO never has content sent externally":      "TestSummarize_NoBYO_ContentOnlyLocal",
+	"Local AI produces the summary":                                     "TestSummarize_LocalSucceeds",
+	"Local AI fails and the user has a BYO provider configured":         "TestSummarize_FallsBackToBYO",
+	"Local AI fails and the user has no BYO provider":                   "TestSummarize_LocalFailsNoBYO_NoExternalSend",
+	"A user without BYO never has content sent externally":              "TestSummarize_NoBYO_ContentOnlyLocal",
+	"A model returns unparseable output and the next endpoint succeeds": "TestSummarize_FallsBackOnMalformedOutput",
 
 	// landing_page.feature
 	"An unauthenticated visit to the root is sent to the welcome page":    "TestUnauthenticatedRootRedirectsToWelcome",