fix: a URL rule's address is capped at 2048 bytes (WR-23)

Rule.Problems had no length check, so a long address reached the
(url, match) UNIQUE btree index and failed with a 500. It now refuses an
address over models.MaxRuleURL with rules.url_invalid, like redirect
sources.

models/rule.go

@@ -96,14 +96,20 @@ func NormalizeMatch(m string) string {
 	return MatchExact
 }
 
-// Problems refuses an address that does not start with "/" or carries * in
-// an exact rule, and an address and match another rule already has (db is
-// the save transaction; nil skips the uniqueness check).
+// MaxRuleURL caps a URL rule's address in bytes, like MaxRedirectURL: the
+// (url, match) UNIQUE btree index refuses rows over about 2.7 KB with a
+// database error (WR-23).
+const MaxRuleURL = 2048
+
+// Problems refuses an address that does not start with "/", is longer than
+// MaxRuleURL or carries * in an exact rule, and an address and match
+// another rule already has (db is the save transaction; nil skips the
+// uniqueness check).
 func (r Rule) Problems(db *gorm.DB) map[string][]Problem {
 	out := map[string][]Problem{}
 	url := strings.TrimSpace(r.URL)
 	match := NormalizeMatch(r.Match)
-	if !strings.HasPrefix(url, "/") || strings.HasPrefix(url, "//") || strings.ContainsAny(url, " \t\r\n") || (match == MatchExact && strings.Contains(url, "*")) {
+	if len(url) > MaxRuleURL || !strings.HasPrefix(url, "/") || strings.HasPrefix(url, "//") || strings.ContainsAny(url, " \t\r\n") || (match == MatchExact && strings.Contains(url, "*")) {
 		out["url"] = []Problem{{Key: errRuleURLInvalid}}
 		return out
 	}