feat(tts): add Doubao Speech provider

Add Volcengine Doubao Speech 2.0 as a TTS provider for Mandarin narration with async timestamp metadata, provider documentation, and setup guidance.

docs/PROVIDERS.md

@@ -39,6 +39,8 @@ GOOGLE_API_KEY=              # Google TTS + Google Imagen
 ELEVENLABS_API_KEY=          # TTS, music, sound effects (10K chars/month free)
 OPENAI_API_KEY=              # OpenAI TTS + DALL-E 3 images
 XAI_API_KEY=                 # xAI Grok image generation/editing + Grok video generation
+DOUBAO_SPEECH_API_KEY=       # Volcengine Doubao Speech TTS (strong Mandarin narration)
+DOUBAO_SPEECH_VOICE_TYPE=    # Default Doubao speaker/voice type
 
 # MULTI-MODEL GATEWAY (one key, 6+ tools)
 FAL_KEY=                     # FLUX, Recraft, Kling, Veo, MiniMax video
@@ -159,6 +161,52 @@ No subscription — pure pay-as-you-go, no minimum spend.
 
 ---
 
+### Doubao Speech — Mandarin TTS
+
+> **Strong Mandarin narration.** Volcengine Doubao Speech is a good choice for Chinese explainer voiceovers and long-form narration that needs subtitle timing metadata.
+
+**Tools unlocked:** `doubao_tts`
+**Env vars:** `DOUBAO_SPEECH_API_KEY`, `DOUBAO_SPEECH_VOICE_TYPE`
+
+#### Setup
+
+1. Open the Volcengine Doubao Speech console and enable Speech Synthesis 2.0.
+2. Create a new-console API Key.
+3. Choose a Speech 2.0 voice type, for example `zh_female_vv_uranus_bigtts`.
+4. Add to `.env`:
+   ```bash
+   DOUBAO_SPEECH_API_KEY=your-api-key
+   DOUBAO_SPEECH_VOICE_TYPE=zh_female_vv_uranus_bigtts
+   ```
+
+#### API Notes
+
+OpenMontage uses the new-console API key flow:
+
+```text
+X-Api-Key: ${DOUBAO_SPEECH_API_KEY}
+X-Api-Resource-Id: seed-tts-2.0
+```
+
+Do not pass a new-console API Key as `X-Api-App-Id` or `X-Api-Access-Key`. That mismatch can produce `load grant: requested grant not found`.
+
+#### What It Is Best For
+
+- Natural Mandarin narration for Chinese-language explainers
+- Async long-form narration via `/api/v3/tts/submit` and `/api/v3/tts/query`
+- Character-level timing metadata for subtitle alignment
+- Calm educational pacing where the video duration can follow the approved voice rhythm
+
+#### Pacing
+
+Start with `speech_rate: 0` for natural Mandarin delivery. If the approved format needs a tighter runtime, compare short samples at `speech_rate: 25` or `50` before generating the full narration. Do not force Doubao to match another provider's duration unless the user explicitly wants that tradeoff.
+
+#### Pricing
+
+Doubao Speech 2.0 is billed by character package or usage in Volcengine. OpenMontage estimates cost from text length and prefers provider-returned usage metadata when available.
+
+---
+
 ### Google — TTS + Imagen (Shared Key)
 
 > **One key, two tools.** Google Cloud TTS has 700+ voices in 50+ languages — the strongest localization option. Imagen 4 generates high-quality images.