feat(tts): wire tenant timeout + fix Gemini text-only 400

- HTTP synthesize + test-connection now read tenant tts.timeout_ms
  (default 120s, was hardcoded 15s/10s). Gemini client default also
  bumped 30s→120s so both layers align when tenant config unset.
- Inline prefix "Speak naturally: " prepended to single-voice text;
  multi-speaker transcripts pass through unchanged.
- ErrTextOnlyResponse sentinel for 400 "text generation" bodies;
  single-voice retries once with stronger prefix. Narrowed needle
  list avoids false positives on unrelated 400s.
- SynthesizeWithFallbackAdapted now returns errors.Join so sentinel
  survives fallback chain; HTTP 422 mapping + locale-translated
  ForLLM in agent tool (EN/VI/ZH catalogs).
- Default Gemini model bumped to gemini-3.1-flash-tts-preview.

docs/codebase-summary.md

@@ -100,7 +100,7 @@ old clients see flat keys; new clients see the full params blob.
 
 ### Gemini Specifics
 
-- Models: `gemini-2.5-flash-preview-tts`, `gemini-2.5-pro-preview-tts` (preview).
+- Models: `gemini-3.1-flash-tts-preview` (default), `gemini-2.5-flash-preview-tts`, `gemini-2.5-pro-preview-tts` (preview).
 - Multi-speaker: up to 2 simultaneous speakers, each with distinct voice + name annotation.
 - Audio tags: inline `<say-as>` / style directives via bracketed prompts.
 - Sentinel errors: `ErrInvalidVoice`, `ErrInvalidModel`, `ErrSpeakerLimit` → HTTP 422 with i18n message.
@@ -143,6 +143,7 @@ Native `image_generation` support in the Codex provider (`POST /codex/responses`
 ## Key Conventions
 
 - **Store layer:** Interface-based; PG (`store/pg/`) + SQLite (`store/sqlitestore/`). Raw SQL, `$1/$2` params.
+- **Session token display:** v3 compaction now uses dynamic max_tokens; session token display reads from `sessions.metadata.last_prompt_tokens`.
 - **Context propagation:** `store.WithLocale`, `store.WithUserID`, `store.WithTenantID`, etc.
 - **Security logs:** `slog.Warn("security.*")` for all security events.
 - **SSRF prevention:** `validateProviderURL()` in `internal/http/tts_validate.go`.