feat: add AI deadline extraction and case summary endpoints (Phase 2J)

Add two Claude API-powered endpoints:
- POST /api/ai/extract-deadlines: accepts PDF upload or JSON text, extracts
  legal deadlines using Claude tool_use for structured output
- POST /api/ai/summarize-case: generates AI summary from case events/deadlines,
  caches result in cases.ai_summary

New files:
- internal/services/ai_service.go: AIService with Anthropic SDK integration
- internal/handlers/ai.go: HTTP handlers for both endpoints
- internal/services/ai_service_test.go: tool schema and serialization tests

Uses anthropic-sdk-go v1.27.1 with Claude Sonnet 4.5. AI service is optional —
endpoints only registered when ANTHROPIC_API_KEY is set.

backend/internal/handlers/helpers.go

@@ -83,3 +83,8 @@ func handleTenantError(w http.ResponseWriter, err error) {
 func parsePathUUID(r *http.Request, key string) (uuid.UUID, error) {
 	return uuid.Parse(r.PathValue(key))
 }
+
+// parseUUID parses a UUID string
+func parseUUID(s string) (uuid.UUID, error) {
+	return uuid.Parse(s)
+}