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)

app/db.py

@@ -0,0 +1,41 @@
+"""Database engine & session factory (PostgreSQL 17 + pgvector).
+
+The engine is created lazily: importing this module never opens a
+connection, so the app boots (and ``/api/health`` reports) even when the
+database is momentarily unavailable.
+"""
+from __future__ import annotations
+
+from collections.abc import Generator
+
+from sqlalchemy import create_engine, text
+from sqlalchemy.orm import DeclarativeBase, Session, sessionmaker
+
+from app.config import get_settings
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+engine = create_engine(get_settings().database_url, pool_pre_ping=True, future=True)
+SessionLocal = sessionmaker(bind=engine, autoflush=False, expire_on_commit=False, future=True)
+
+
+def get_db() -> Generator[Session, None, None]:
+    """FastAPI dependency yielding a database session."""
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()
+
+
+def db_available() -> bool:
+    """Cheap liveness probe used by ``/api/health``."""
+    try:
+        with engine.connect() as conn:
+            conn.execute(text("SELECT 1"))
+        return True
+    except Exception:
+        return False