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/appointments.go

@@ -203,14 +203,3 @@ func (h *AppointmentHandler) Delete(w http.ResponseWriter, r *http.Request) {
 	w.WriteHeader(http.StatusNoContent)
 }
 
-func writeJSON(w http.ResponseWriter, status int, v any) {
-	w.Header().Set("Content-Type", "application/json")
-	w.WriteHeader(status)
-	json.NewEncoder(w).Encode(v)
-}
-
-func writeError(w http.ResponseWriter, status int, msg string) {
-	w.Header().Set("Content-Type", "application/json")
-	w.WriteHeader(status)
-	json.NewEncoder(w).Encode(map[string]string{"error": msg})
-}