feat(config): migrate runtime configuration to YAML-only

[Intrinsical-AI]

Summary

This PR replaces env-based runtime configuration with a single config.yaml source of truth.

Settings remains as a Pydantic validation layer, but it no longer reads from environment variables or .env files. The app now loads configuration from YAML at startup, resolves relative paths against the YAML file location, and shares the same config model across HTTP, CLI, and bootstrap flows.

What changed

• Added config.yaml as the canonical runtime config.
• Added config.example.yaml as the documented template.
• Reworked Settings to validate YAML input instead of reading env vars.
• Removed runtime env-based config reads across:
  • embedder/cache wiring
  • blocking worker/queue settings
  • metrics and lock paths
  • eval dataset config
  • OpenRouter and retrieval-related validation messages
• Updated docs to point to config.yaml and config.example.yaml.
• Updated tests to cover loader behavior, path resolution, and runtime wiring.

Validation

• uv run ruff check src/local_rag_backend tests
• uv run pytest -q -o addopts=''
• Result: 679 passed, 4 skipped

Notes

• This is an operational breaking change: runtime config must now live in config.yaml.
• No env/.env back-compat is retained by design.
• Relative paths in config are resolved relative to the YAML file, not the current working directory.

[Intrinsical-AI]

Friendly reminder:

BREAKING CHANGE