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.

app/models/migrations/__init__.py

@@ -0,0 +1,12 @@
+"""SQL migration files applied by :mod:`app.db` at startup.
+
+This package holds the authoritative schema history for the
+``chicken_babies_site`` database. Each ``.sql`` file is applied exactly
+once in lexicographic order; the runner tracks which files have been
+applied in a ``schema_migrations`` table.
+
+No Python code lives here — the files are trusted, developer-authored
+SQL loaded via ``sqlite3.Connection.executescript`` at boot.
+"""
+
+from __future__ import annotations