Move schema_migrations table creation into 000.sql with INTEGER version column

Refactors the migration system to follow the pixa pattern:

- Add 000.sql bootstrap migration that creates schema_migrations with
  INTEGER PRIMARY KEY version column
- Go code no longer creates the migrations table inline; it reads and
  executes 000.sql as a bootstrap step before the normal migration loop
- Export ParseMigrationVersion and ApplyMigrations for test use
- Add legacy TEXT-to-INTEGER conversion for existing databases that
  stored migration versions as filenames (e.g. '001_initial.sql')
- Wrap individual migration application in transactions for safety
- Add comprehensive tests for version parsing, fresh database bootstrap,
  idempotent re-application, and legacy conversion

internal/database/migrations/000.sql

@@ -0,0 +1,9 @@
+-- Migration 000: Schema migrations tracking table
+-- Applied as a bootstrap step before the normal migration loop.
+
+CREATE TABLE IF NOT EXISTS schema_migrations (
+    version INTEGER PRIMARY KEY,
+    applied_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+INSERT OR IGNORE INTO schema_migrations (version) VALUES (0);