Escape the database path in the SQLite connection string (closes #518)

SQLite reads a file: URI's path only up to the first ? or #, and
decodes %XX escapes in it. With a data directory such as
/srv/hooks#1, every database opened the one file /srv/hooks. The
connection string is now built with url.URL, which escapes the path,
and OmitHost keeps a relative path relative.

Tests open databases in a directory whose name holds #, ? and %, and
at a relative path, and check through PRAGMA database_list that each
opened the file asked for. A handlers test comment that blamed # in
the temporary directory's path is dropped, as that is no longer true.

Model: opus-5-5

internal/handlers/delivery_attempts_test.go

@@ -42,9 +42,6 @@ func TestDeliveryAttempts_ReadInTheTargetTypesOwnTerms(t *testing.T) {
 		{database.TargetTypeLog, true, 0, "", "written to the log", ""},
 	}
 
-	// Named for type and outcome, so that no two cases share a name:
-	// Go adds "#01" to a repeated one, and a "#" in the temporary
-	// directory's path cuts the path SQLiteDSN gives SQLite short.
 	for _, tc := range cases {
 		t.Run(string(tc.targetType)+" "+tc.outcome, func(t *testing.T) {
 			t.Parallel()