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/embed.go

@@ -0,0 +1,10 @@
+// Package migrations embeds the SQL schema migrations applied at boot by
+// internal/store. Files are named NNNN_slug.sql and applied in filename order.
+package migrations
+
+import "embed"
+
+// FS holds every *.sql migration in this directory.
+//
+//go:embed *.sql
+var FS embed.FS