Add OpenAI-compatible agent profile

[WarGloom]

This is the focused follow-up from the closed local-agent setup PR. It only adds the generic openai_compatible profile path, so BrainDB can point the internal agent at OpenAI-compatible /v1 endpoints without adding another provider abstraction.

What changed:

• add openai_compatible to _LLM_PROFILES and let AGENT_BASE_URL override the profile base URL
• pass AGENT_BASE_URL and AGENT_API_KEY into the API container
• document the env vars with an Ollama-style example using the generic profile name
• add unit coverage for the new profile and existing profile regressions

What this does not include:

• no local_ollama alias
• no Codex profile or OPENAI_API_KEY passthrough
• no fast-path save routing
• no bundled Postgres, manage script, or GPU compose changes

Validation:

• python -m pytest tests/test_config_profiles.py -q -> 7 passed
• docker compose config
• python -m py_compile braindb/config.py tests/conftest.py tests/test_config_profiles.py
• manual Settings smoke with LLM_PROFILE=openai_compatible, AGENT_MODEL=openai/llama3.2:3b, AGENT_BASE_URL=http://localhost:11434/v1

I could not run the full suite in this shell because the local Python environment is missing agents and psycopg2; the targeted config tests above do not depend on those packages.