feat: phase 2 content model + cache — SQLite schema, markdown, TTL

Stand up the full SQLite content layer: all 7 tables from the authoritative
schema with WAL + foreign-keys enforced per-connection, entity dataclasses
plus row mappers, hand-rolled versioned migrations tracked in
schema_migrations, and an idempotent Python seed (system user + welcome
post + About page).

Add a Markdown->HTML service using markdown-it-py with a strict bleach
allowlist (tables intentionally omitted on both sides). Add a typed
in-process TTLCache[K,V] and wire it into real DB-backed PostService and
PageService, both exposing invalidate_all() for Phase 4 admin writes.

Rewire / and /about to read from the DB; homepage renders the seeded
welcome post, About renders page.title + sanitized body_html_cached.
Update the Phase 1 route tests accordingly.

Mark Phase 2 complete in docs/ROADMAP.md.

tests/conftest.py

@@ -0,0 +1,56 @@
+"""Shared pytest fixtures for the ``chicken_babies_site`` suite.
+
+Key fixtures:
+
+- ``db_engine``: a session-scoped SQLAlchemy engine pointed at a
+  temp-file SQLite database. Migrations + seed run once per test
+  session. Per the CLAUDE.md mandate, tests do NOT mock the DB —
+  they use a real SQLite file so behavior matches production.
+- ``clean_db_engine``: a function-scoped engine with migrations
+  applied but seed NOT run, for tests that need to exercise the
+  first-boot path.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Iterator
+
+import pytest
+from sqlalchemy import Engine
+
+from app.db import build_engine, run_migrations
+from app.models.seed import run_seed
+
+
+@pytest.fixture(scope="session")
+def db_engine(tmp_path_factory: pytest.TempPathFactory) -> Iterator[Engine]:
+    """Return a migrated + seeded SQLite engine shared across the session.
+
+    Uses a real on-disk file (NOT ``:memory:``) because the CLAUDE.md
+    project rules forbid mocking the DB in auth / magic-link tests,
+    and doing the same here keeps the behavior identical to
+    production.
+    """
+    db_path: Path = tmp_path_factory.mktemp("db") / "test.db"
+    engine = build_engine(f"sqlite:///{db_path}")
+    run_migrations(engine)
+    run_seed(engine)
+    yield engine
+    engine.dispose()
+
+
+@pytest.fixture
+def clean_db_engine(tmp_path: Path) -> Iterator[Engine]:
+    """Return a fresh engine with tables created but NO seed data.
+
+    Function-scoped so each test that uses it starts with a virgin
+    database — useful for asserting first-run seed behavior and
+    migration idempotency without contaminating the session-scoped
+    ``db_engine``.
+    """
+    db_path = tmp_path / "clean.db"
+    engine = build_engine(f"sqlite:///{db_path}")
+    run_migrations(engine)
+    yield engine
+    engine.dispose()