feat(ai): LlmConfig loaded from llm.toml with legacy env overrides

Adds ai::config: one serde struct for every LLM setting (backend, llama.cpp,
Ollama, OpenRouter, embedding, TTS, agentic caps), resolved as defaults ->
llm.toml (LLM_CONFIG_PATH, default ./llm.toml) -> the old env vars. Env
overrides are kept for one release and listed in a single startup
deprecation warning; LLAMA_SWAP_ALLOWED_MODELS maps to picker.include so
current deploys keep their picker. Unknown keys, zero timeouts and
backend=llamacpp without a URL fail startup. OPENROUTER_API_KEY stays
env-only.

llm.example.toml documents the schema and is parsed by a test. main loads
the config right after tracing init; readers switch over next.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -25,6 +25,7 @@ futures = "0.3.5"
 jsonwebtoken = "9.3.0"
 serde = "1"
 serde_json = "1"
+toml = "0.9"
 diesel = { version = "2.2.10", features = ["sqlite", "64-column-tables"] }
 libsqlite3-sys = "0.35"
 diesel_migrations = "2.2.0"