Add ReDoS validation for SpamRule.pattern field (FRE-4512)

- Create regex-validation utility with ReDoS detection (nested quantifiers,
  overlapping alternations, complexity limits)
- Add @db.VarChar(500) constraint on pattern field in Prisma schema
- Integrate validation in rule-engine at load time and evaluation time
- Add 46 unit tests covering syntax, ReDoS patterns, complexity, edge cases

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/db/prisma/schema.prisma

@@ -309,7 +309,7 @@ model SpamCallAnalysis {
 model SpamRule {
   id          String       @id @default(uuid())
   name        String       @unique
-  pattern     String // Regex pattern - needs ReDoS validation
+  pattern     String       @db.VarChar(500) // Regex pattern - validated for ReDoS at application layer
   decision    SpamDecision
   description String?
   isActive    Boolean      @default(true)