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/llm/client.go

@@ -34,15 +34,35 @@ type Client struct {
 	httpClient *http.Client
 }
 
+// Option configures a Client at construction. Variadic so the existing 4-arg
+// call sites stay valid as new knobs are added.
+type Option func(*Client)
+
+// WithMaxTokens overrides the per-request completion budget. The summarizer uses
+// this to cap completion for small-context models (e.g. koala/phi4-mini, 8k):
+// with the default 8192 budget, prompt + max_tokens overflows an 8k context and
+// the gateway returns HTTP 400. A non-positive n is ignored (keeps the default).
+func WithMaxTokens(n int) Option {
+	return func(c *Client) {
+		if n > 0 {
+			c.maxTokens = n
+		}
+	}
+}
+
 // New constructs a Client.
-func New(baseURL, apiKey, model string, timeout time.Duration) *Client {
-	return &Client{
+func New(baseURL, apiKey, model string, timeout time.Duration, opts ...Option) *Client {
+	c := &Client{
 		baseURL:    strings.TrimRight(baseURL, "/"),
 		apiKey:     apiKey,
 		model:      model,
 		maxTokens:  defaultMaxTokens,
 		httpClient: &http.Client{Timeout: timeout},
 	}
+	for _, opt := range opts {
+		opt(c)
+	}
+	return c
 }
 
 type chatRequest struct {