Add SQLite store and embedded migration runner

internal/store opens $MEALQUEST_DATA_DIR/mealquest.db with modernc.org/sqlite
(CGO-free) as one writer connection and a read-only pool of four, with
busy_timeout=5000, journal_mode=WAL and foreign_keys=ON on every connection.

migrations/*.sql are embedded and applied at boot in filename order, each
in its own transaction together with its schema_migrations row, so a failing
migration rolls back and fails boot. A database holding a version this build
does not know is refused. 0001_init.sql is a placeholder until the v1 schema
lands.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/0001_init.sql

@@ -0,0 +1,2 @@
+-- 0001_init: placeholder so the migrations embed compiles.
+-- Replaced by the full v1 schema in Feature 02 task 3.