fix: abort startup on explicitly empty db_url instead of deriving

The derived file:...state.sqlite3 URL is a default and defaults apply
only to omitted keys: db_url set to an empty string now aborts naming
the key, matching the existing behavior of state_dir: "".

internal/config/config.go

@@ -113,9 +113,19 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 			"upstream_connections_per_host", DefaultUpstreamConnectionsPerHost),
 	}
 
-	// Build DBURL from StateDir if not explicitly set
+	// Build DBURL from StateDir if not explicitly set. The derived URL
+	// is a default: it applies only when db_url is omitted, never to an
+	// explicitly empty value.
 	c.DBURL = loader.stringVal("db_url", "")
-	if c.DBURL == "" {
+	if c.DBURL == "" && loader.err == nil {
+		if sc != nil {
+			if _, present := sc.Get("db_url"); present {
+				return nil, fmt.Errorf(
+					"config key %q: value must not be empty; omit the key to derive it from state_dir",
+					"db_url")
+			}
+		}
+
 		c.DBURL = fmt.Sprintf("file:%s/state.sqlite3?_journal_mode=WAL", c.StateDir)
 	}