Create new databases with auto_vacuum incremental (closes #43)

SQLite only accepts auto_vacuum before the database file is first
written. The connection switched to WAL first, which writes the file,
so the PRAGMA auto_vacuum in Initialize came too late and was ignored:
new databases had auto_vacuum off and the periodic incremental vacuum
freed nothing. The setting now goes in the connection string, which the
driver applies on open before the journal mode, and the late PRAGMA is
removed. Tests check that every pooled connection sees auto_vacuum
incremental on a new database and that Vacuum reduces the free page
count after routes are deleted.

Model: opus-5-5

internal/database/database.go

@@ -94,8 +94,13 @@ func New(cfg *config.Config, logger *logger.Logger) (*Database, error) {
 	// a transaction that reads before writing starts as a reader and, when it
 	// then writes while another connection holds the write lock, fails at once
 	// with "database is locked" without waiting for _busy_timeout.
+	// _auto_vacuum=incremental lets Vacuum return free pages to the filesystem.
+	// SQLite only accepts it before the database file is first written, and the
+	// switch to WAL writes it, so it must be here: the driver applies it on open,
+	// before _journal_mode. On an existing file it changes nothing.
 	dsn := fmt.Sprintf(
-		"file:%s?_cache_size=%d&_synchronous=OFF&_busy_timeout=%d&_journal_mode=WAL&_txlock=immediate",
+		"file:%s?_cache_size=%d&_synchronous=OFF&_busy_timeout=%d"+
+			"&_auto_vacuum=incremental&_journal_mode=WAL&_txlock=immediate",
 		dbPath,
 		sqliteCacheSizeKiB,
 		sqliteBusyTimeoutMs,
@@ -140,9 +145,8 @@ func (d *Database) Initialize() error {
 	// heap. The heap limits below are process-wide, so setting them once here is
 	// enough for the whole pool.
 	pragmas := []string{
-		"PRAGMA journal_mode=WAL",        // Write-Ahead Logging
-		"PRAGMA analysis_limit=0",        // Disable automatic ANALYZE
-		"PRAGMA auto_vacuum=INCREMENTAL", // Enable incremental vacuum
+		"PRAGMA journal_mode=WAL", // Write-Ahead Logging
+		"PRAGMA analysis_limit=0", // Disable automatic ANALYZE
 		fmt.Sprintf("PRAGMA soft_heap_limit=%d", sqliteSoftHeapLimitBytes),
 		fmt.Sprintf("PRAGMA hard_heap_limit=%d", sqliteHardHeapLimitBytes),
 	}