feat: scaffold Brain of Reese — FastAPI RAG chat over Postgres 17 + pgvector

Foundation (phase 01, verified):
- FastAPI app: /api/health, /api/suggestions, /api/chat (placeholder),
  static frontend served locally (no CDN)
- Postgres 17 + pgvector via db/Containerfile + compose.yaml
  (podman compose up -d db), Alembic initial migration (documents,
  chunks with vector(768), query_log)
- LLM client targeting https://aipi.reeseapps.com/v1 (turbo/embed);
  scripts/llm_probe.py verified models + 768-dim embeddings live
- Conditional debugpy: imported only when DEBUGPY=1 (attach on demand,
  :5678); logging config for clean single-line logs
- Frontend shell: mobile-first chat + Sources pages, tokens, a11y baselines
- Tests: 24 unit+integration (99% coverage on app/), ruff + pyright clean,
  Playwright smoke E2E (3 tests) against a deterministic mock LLM
- Planning: .agent/PLAN.md (architecture + LOCKED decisions), AGENTS.md,
  6 user stories, 7 phase files (one story / one phase / one Playwright
  suite each)

tests/unit/test_config.py

@@ -0,0 +1,42 @@
+"""Unit tests: settings defaults & env overrides."""
+from __future__ import annotations
+
+from typing import Any
+
+from app.config import Settings
+
+
+def _settings(**kwargs: Any) -> Settings:
+    """Build Settings without reading a .env file (deterministic tests)."""
+    kwargs.setdefault("_env_file", None)
+    return Settings(**kwargs)  # pyright: ignore[reportCallIssue]  (kwarg exists at runtime)
+
+
+def test_defaults_match_locked_decisions() -> None:
+    s = _settings()
+    assert s.llm_chat_model == "turbo"
+    assert s.llm_embed_model == "embed"
+    assert s.embedding_dim == 768
+    assert s.llm_base_url.endswith("/v1")
+    assert 0 < s.relevance_threshold < 1
+    assert s.top_k_chunks >= 1
+    assert s.top_n_docs >= 1
+    assert len(s.suggestions) >= 3
+
+
+def test_env_override(monkeypatch) -> None:
+    monkeypatch.setenv("BOR_RELEVANCE_THRESHOLD", "0.42")
+    monkeypatch.setenv("BOR_LLM_CHAT_MODEL", "juggernaut")
+    s = _settings()
+    assert s.relevance_threshold == 0.42
+    assert s.llm_chat_model == "juggernaut"
+
+
+def test_effective_api_key_fallback(monkeypatch) -> None:
+    monkeypatch.delenv("AIPI_KEY", raising=False)
+    s = _settings()
+    assert s.effective_api_key == "not-needed"
+
+    monkeypatch.setenv("AIPI_KEY", "sk-from-env")
+    s2 = _settings()
+    assert s2.effective_api_key == "sk-from-env"