Move schema_migrations table creation into 000.sql with INTEGER version column (#172)

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>

internal/database/migrations_test.go

@@ -0,0 +1,117 @@
+package database_test
+
+import (
+	"context"
+	"database/sql"
+	"testing"
+
+	_ "github.com/mattn/go-sqlite3"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/upaas/internal/database"
+)
+
+func TestParseMigrationVersion(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		filename    string
+		wantVersion int
+		wantErr     bool
+	}{
+		{filename: "000_migration.sql", wantVersion: 0},
+		{filename: "001_initial.sql", wantVersion: 1},
+		{filename: "002_remove_container_id.sql", wantVersion: 2},
+		{filename: "007_add_resource_limits.sql", wantVersion: 7},
+		{filename: "100_large_version.sql", wantVersion: 100},
+		{filename: ".sql", wantErr: true},
+		{filename: "_foo.sql", wantErr: true},
+		{filename: "abc_foo.sql", wantErr: true},
+		{filename: "1a2_bad.sql", wantErr: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.filename, func(t *testing.T) {
+			t.Parallel()
+
+			version, err := database.ParseMigrationVersion(tt.filename)
+			if tt.wantErr {
+				assert.Error(t, err)
+
+				return
+			}
+
+			require.NoError(t, err)
+			assert.Equal(t, tt.wantVersion, version)
+		})
+	}
+}
+
+func TestApplyMigrationsFreshDatabase(t *testing.T) {
+	t.Parallel()
+
+	db, err := sql.Open("sqlite3", ":memory:?_foreign_keys=on")
+	require.NoError(t, err)
+
+	defer func() { _ = db.Close() }()
+
+	ctx := context.Background()
+
+	err = database.ApplyMigrations(ctx, db, nil)
+	require.NoError(t, err)
+
+	// Verify schema_migrations table exists with INTEGER version column.
+	var version int
+
+	err = db.QueryRowContext(ctx,
+		"SELECT version FROM schema_migrations WHERE version = 0",
+	).Scan(&version)
+	require.NoError(t, err)
+	assert.Equal(t, 0, version)
+
+	// Verify that all migrations were recorded.
+	var count int
+
+	err = db.QueryRowContext(ctx,
+		"SELECT COUNT(*) FROM schema_migrations",
+	).Scan(&count)
+	require.NoError(t, err)
+	// 000 bootstrap + 001 through 007 = 8 entries.
+	assert.Equal(t, 8, count)
+
+	// Verify application tables were created by the migrations.
+	var tableCount int
+
+	err = db.QueryRowContext(ctx,
+		"SELECT COUNT(*) FROM sqlite_master WHERE type='table' AND name='users'",
+	).Scan(&tableCount)
+	require.NoError(t, err)
+	assert.Equal(t, 1, tableCount)
+}
+
+func TestApplyMigrationsIdempotent(t *testing.T) {
+	t.Parallel()
+
+	db, err := sql.Open("sqlite3", ":memory:?_foreign_keys=on")
+	require.NoError(t, err)
+
+	defer func() { _ = db.Close() }()
+
+	ctx := context.Background()
+
+	// Apply twice — second run should be a no-op.
+	err = database.ApplyMigrations(ctx, db, nil)
+	require.NoError(t, err)
+
+	err = database.ApplyMigrations(ctx, db, nil)
+	require.NoError(t, err)
+
+	var count int
+
+	err = db.QueryRowContext(ctx,
+		"SELECT COUNT(*) FROM schema_migrations",
+	).Scan(&count)
+	require.NoError(t, err)
+	assert.Equal(t, 8, count)
+}