feat: vet OpenRouter as a second BYOK embedding provider

[EthanThatOneKid]

What

BYOK was hard-wired to OpenAI. This makes the provider a property of the model and adds OpenRouter as a vetted second provider.

• catalog.ts: provider table (EMBEDDING_PROVIDERS, fixed endpoint per provider, still no customer-supplied host); new models openrouter/openai/text-embedding-3-small and -large (dimensions pinned to 1536).
• openai.ts becomes provider.ts; the client picks its endpoint from the model's provider.
• A namespace can hold one credential per provider. The decrypted-key cache is now keyed by namespace and provider (it was keyed by namespace only, which would have crossed providers). Revoking one provider only stales worlds on that provider's models.
• World create/PATCH and reindex check the credential of the chosen model's provider, not just "any credential".
• GET/DELETE /namespaces/{ns}/embedding-credential take an optional ?provider=. It may be omitted while one live credential exists (existing callers keep working); with two live credentials it returns 400 until provider is given.
• Status lookup in store.ts no longer hard-codes provider = 'openai'.

Contract change and dependents

Additive OpenAPI change: new provider and embeddingModel enum values, new optional query parameter, new 400 on GET/DELETE.
Dependents, in merge order: this PR -> worlds-client-ts (regenerate and publish) -> wazoo-api round-trip script (add a provider switch) -> wazoo-console if it lists providers.

Not observed

Tests use a fake fetch; I have not called OpenRouter. The OpenRouter embeddings endpoint is OpenAI-format and its model list includes openai/text-embedding-3-small/large, but that dimensions: 1536 is honoured through OpenRouter is unverified until the QA round-trip runs with a real OpenRouter key.

Checks

230 tests pass (new: OpenRouter client, per-provider keys and cache, scoped revoke, ambiguous GET/DELETE, PATCH needs the model's provider credential), tsc, prettier, OpenAPI snapshot regenerated.

🤖 Generated with Claude Code