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/api/chat.py

@@ -0,0 +1,29 @@
+"""POST /api/chat — placeholder (phase 01).
+
+Phase 03 replaces this with the real RAG pipeline and SSE streaming
+(PLAN §4 contract: ``delta`` events + final ``done``). The placeholder
+keeps the same JSON shape the frontend already consumes, so the UI round-trip
+is exercised end-to-end from day one.
+"""
+from __future__ import annotations
+
+from fastapi import APIRouter
+
+from app.schemas import ChatRequest
+
+router = APIRouter()
+
+
+@router.post("/chat")
+async def chat(request: ChatRequest) -> dict[str, object]:
+    """Placeholder answer — no LLM, no DB."""
+    return {
+        "ok": True,
+        "answer": (
+            "Hey! My neurons are still wiring up — the real Brain "
+            "(RAG over your docs, powered by aipi) lands in the next "
+            "phases. Try me again soon! 🧠"
+        ),
+        "deflected": False,
+        "sources": [],
+    }