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)

db/Containerfile

@@ -0,0 +1,19 @@
+# Postgres 17 + pgvector.
+# Base image is the official docker.io/postgres:17; pgvector is compiled in
+# at build time so the app gets native `vector` type + cosine (`<=>`) search.
+FROM docker.io/postgres:17
+
+ARG PGVECTOR_VERSION=v0.8.0
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+         build-essential git ca-certificates postgresql-server-dev-17 \
+    && git clone --branch ${PGVECTOR_VERSION} --depth 1 https://github.com/pgvector/pgvector.git /tmp/pgvector \
+    && make -C /tmp/pgvector \
+    && make -C /tmp/pgvector install \
+    && rm -rf /tmp/pgvector \
+    && apt-get purge -y --auto-remove -qq \
+         build-essential git ca-certificates postgresql-server-dev-17 \
+    && rm -rf /var/lib/apt/lists/*
+
+EXPOSE 5432