Closes#171
Refactors the migration system to follow the [pixa pattern](sneak/pixa#36):
## Changes
### New file: `internal/database/migrations/000.sql`
- Bootstrap migration that creates `schema_migrations` with `version INTEGER PRIMARY KEY`
- Self-contained: includes both `CREATE TABLE IF NOT EXISTS` and `INSERT OR IGNORE INTO schema_migrations (version) VALUES (0)`
### Refactored: `internal/database/migrations.go`
- **Go code no longer creates the migrations table inline** — it reads and executes `000.sql` as a bootstrap step before the normal migration loop
- **`ParseMigrationVersion(filename)`** — exported function that extracts integer version from filenames like `001_initial.sql` → `1`
- **`ApplyMigrations(ctx, db, log)`** — exported function for tests to apply schema without full fx lifecycle
- **`bootstrapMigrationsTable`** — checks if table exists; if missing, runs `000.sql`; if present, checks for legacy format
- **`convertLegacyMigrations`** — one-time conversion for existing databases that stored versions as TEXT filenames (e.g. `"001_initial.sql"`) to INTEGER versions (e.g. `1`)
- **Transaction-wrapped migration application** — each migration runs in a transaction for atomicity
- Sentinel error `ErrInvalidMigrationFilename` for static error wrapping
### New file: `internal/database/migrations_test.go`
- `TestParseMigrationVersion` — valid/invalid filename parsing
- `TestApplyMigrationsFreshDatabase` — verifies bootstrap creates table, all migrations apply, application tables exist
- `TestApplyMigrationsIdempotent` — double-apply is a no-op
- `TestApplyMigrationsLegacyConversion` — simulates old TEXT-based table with filename entries, verifies conversion to integer format
## Backward Compatibility
Existing databases with the old TEXT-based `schema_migrations` table are automatically converted on first run. The conversion:
1. Detects legacy entries (versions containing `_`)
2. Parses integer version from each legacy filename
3. Drops the old table
4. Creates the new INTEGER-based table via `000.sql`
5. Re-inserts all parsed integer versions
All existing migrations (001-007) continue to work unchanged.
Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: clawbot <clawbot@sneak.berlin>
Reviewed-on: #172
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>
