Escape state_dir in the db_url derived from it (closes #290)

The derived db_url put state_dir into a file: URL as it was, so SQLite
decoded a %HH in the name and ended the path at a ? or #, and the
journal mode parameter was lost. It is now built with net/url, which
percent-encodes the directory name; SQLite decodes it back and opens
state.sqlite3 in that directory. OmitHost keeps the usual form
file:/var/lib/pixa/state.sqlite3 and a relative state_dir a path.

Model: opus-5-5

internal/config/config.go

@@ -338,8 +338,7 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 				settingName(keyDBURL), errValueEmpty)
 		}
 
-		// The driver sets the journal mode only through a _pragma parameter.
-		c.DBURL = fmt.Sprintf("file:%s/state.sqlite3?_pragma=journal_mode(WAL)", c.StateDir)
+		c.DBURL = defaultDBURL(c.StateDir)
 	}
 
 	if loader.err != nil {
@@ -354,6 +353,24 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 	return c, nil
 }
 
+// defaultDBURL returns the db_url used when db_url is omitted: the file
+// state.sqlite3 in stateDir, in WAL mode, which the driver sets only
+// through a _pragma parameter. String() percent-encodes any %, ? or # in
+// the directory name, which SQLite decodes back, so the URL names exactly
+// this file. OmitHost writes file:/var/lib/pixa/... instead of
+// file:///var/lib/pixa/..., and keeps a relative stateDir from being read
+// as a host name.
+func defaultDBURL(stateDir string) string {
+	dbURL := url.URL{
+		Scheme:   "file",
+		OmitHost: true,
+		Path:     stateDir + "/state.sqlite3",
+		RawQuery: "_pragma=journal_mode(WAL)",
+	}
+
+	return dbURL.String()
+}
+
 // validateKnownKeys rejects configuration files containing keys the
 // application does not understand, so typos fail at startup instead of
 // being silently ignored, and rejects keys that are explicitly set to