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/router/router.go

@@ -11,7 +11,7 @@ import (
 	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
 )
 
-func New(db *sqlx.DB, authMW *auth.Middleware) http.Handler {
+func New(db *sqlx.DB, authMW *auth.Middleware, anthropicAPIKey string) http.Handler {
 	mux := http.NewServeMux()
 
 	// Services
@@ -24,6 +24,13 @@ func New(db *sqlx.DB, authMW *auth.Middleware) http.Handler {
 	deadlineRuleSvc := services.NewDeadlineRuleService(db)
 	calculator := services.NewDeadlineCalculator(holidaySvc)
 
+	// AI service (optional — only if API key is configured)
+	var aiH *handlers.AIHandler
+	if anthropicAPIKey != "" {
+		aiSvc := services.NewAIService(anthropicAPIKey, db)
+		aiH = handlers.NewAIHandler(aiSvc, db)
+	}
+
 	// Middleware
 	tenantResolver := auth.NewTenantResolver(tenantSvc)
 
@@ -86,6 +93,12 @@ func New(db *sqlx.DB, authMW *auth.Middleware) http.Handler {
 	scoped.HandleFunc("PUT /api/appointments/{id}", apptH.Update)
 	scoped.HandleFunc("DELETE /api/appointments/{id}", apptH.Delete)
 
+	// AI endpoints
+	if aiH != nil {
+		scoped.HandleFunc("POST /api/ai/extract-deadlines", aiH.ExtractDeadlines)
+		scoped.HandleFunc("POST /api/ai/summarize-case", aiH.SummarizeCase)
+	}
+
 	// Placeholder routes for future phases
 	scoped.HandleFunc("GET /api/documents", placeholder("documents"))