docs: audit mdx docs against multi-engine backend (#484)

* docs: audit mdx docs against multi-engine backend and refresh stale content

Rewrote developer-facing docs that predated the TTSBackend Protocol /
ModelConfig registry refactor (architecture, tts-generation,
model-management, transcription). Updated user-facing docs to reflect all
seven shipped engines (Qwen, Qwen CustomVoice, LuxTTS, Chatterbox,
Chatterbox Turbo, TADA, Kokoro) instead of the outdated "5 engines" claim.

Also fixes:
- Stale app identifier (com.voicebox.app → sh.voicebox.app)
- CUDA backend update flow (now two-archive split, not N-way chunks)
- Whisper model list (removed tiny, added turbo)
- Broken /development/ and /guides/ route links
- Stale just commands and install steps (missing --no-deps chatterbox/tada)
- Removed ASCII art diagrams from README and stories.mdx
- History Generation schema sync with DB model

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* docs: add DeepWiki badge to README

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* docs: address PR review feedback

- architecture.mdx: fix backends/ file list (remove nonexistent qwen_backend.py, rename tada_backend.py → hume_backend.py)
- model-management.mdx: Kokoro language count 9 → 8 (matches ModelConfig)
- model-management.mdx: ProgressManager path services/ → utils/
- tts-generation.mdx: ModelConfig example uses field(default_factory=...) — mutable default would raise at runtime
- tts-generation.mdx: "1080p samples" → "on CUDA" (1080p is video, not audio)
- PROJECT_STATUS.md: replace ASCII architecture diagram with prose (matches no-ASCII-art rule)

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* fix(app): guard against undefined engine in FloatingGenerateBox preset check

form.getValues('engine') returns string | undefined; Set<string>.has()
rejects undefined under strict mode. Added a truthy guard before the
preset lookup.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>

CONTRIBUTING.md

@@ -260,7 +260,7 @@ voicebox/
 
 ### ✨ New Features
 
-- Check the roadmap in README.md
+- Check the roadmap in README.md and the engineering status in [`docs/PROJECT_STATUS.md`](docs/PROJECT_STATUS.md) before proposing work — it lists prioritized tasks (Tier 1 → 3), known architectural bottlenecks, and candidate TTS engines already under evaluation (including why some have been backlogged)
 - Discuss major features in an issue first
 - Keep features focused and well-scoped
 
@@ -378,6 +378,7 @@ See [docs/TROUBLESHOOTING.md](docs/TROUBLESHOOTING.md) for common issues and sol
 
 - [README.md](README.md) - Project overview
 - [backend/README.md](backend/README.md) - API documentation
+- [docs/PROJECT_STATUS.md](docs/PROJECT_STATUS.md) - Living engineering roadmap: architecture, shipped vs in-flight work, prioritized open issues, candidate TTS engines under evaluation, architectural bottlenecks. Keep this updated when you ship significant features, close or backlog a model integration, or identify new bottlenecks.
 - [docs/AUTOUPDATER_QUICKSTART.md](docs/AUTOUPDATER_QUICKSTART.md) - Auto-updater setup
 - [SECURITY.md](SECURITY.md) - Security policy
 - [CHANGELOG.md](CHANGELOG.md) - Version history