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/go.mod

@@ -3,8 +3,14 @@ module mgit.msbls.de/m/KanzlAI-mGMT
 go 1.25.5
 
 require (
+	github.com/anthropics/anthropic-sdk-go v1.27.1 // indirect
 	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/jmoiron/sqlx v1.4.0 // indirect
 	github.com/lib/pq v1.12.0 // indirect
+	github.com/tidwall/gjson v1.18.0 // indirect
+	github.com/tidwall/match v1.1.1 // indirect
+	github.com/tidwall/pretty v1.2.1 // indirect
+	github.com/tidwall/sjson v1.2.5 // indirect
+	golang.org/x/sync v0.16.0 // indirect
 )