agent-memory-api: OPENROUTER_BASE hardcoded — same generalization argument as entity-extraction-worker

[jkdavies]

At commit `151a8d1c922ffadad08399508efe46b207a5894e`:

```ts
// integrations/agent-memory-api/index.ts:7-10
const SUPABASE_URL = Deno.env.get("SUPABASE_URL")!;
const SUPABASE_SERVICE_ROLE_KEY = Deno.env.get("SUPABASE_SERVICE_ROLE_KEY")!;
const OPENROUTER_API_KEY = Deno.env.get("OPENROUTER_API_KEY")!;
const MCP_ACCESS_KEY = Deno.env.get("MCP_ACCESS_KEY")!;
const OPENROUTER_BASE = "https://openrouter.ai/api/v1\";
```

Same shape as #313 — the base URL is a string literal where the API key is env-driven. Self-hosted deployments that want to route through LiteLLM (or another OpenAI-compatible gateway) currently have to fork to flip the URL.

Suggested patch

```ts
const OPENROUTER_BASE = Deno.env.get("OPENROUTER_BASE") ?? "https://openrouter.ai/api/v1\";
```

(Or rename to `CHAT_API_BASE` to mirror the MCP server's convention, which already supports env override.)

Default behavior unchanged for OpenRouter-default deployments.

Context

Self-hosted at Davies Farms — see #313 + #314 (the other two surface-area items). All three are the same generalization argument: the upstream default to a specific SaaS endpoint is fine, but a one-line env-override unblocks any non-default deployment without changing default behavior.

Happy to PR all three together as a single "env-overridable base URLs" PR if you'd prefer.