feat: AI-powered features — document drafting, case strategy, similar case finder (P2)

Backend:
- DraftDocument: Claude generates legal documents from case data + template type
  (14 template types: Klageschrift, UPC claims, Abmahnung, etc.)
- CaseStrategy: Opus-powered strategic analysis with next steps, risk assessment,
  and timeline optimization (structured tool output)
- FindSimilarCases: queries youpc.org Supabase for UPC cases, Claude ranks by
  relevance with explanations and key holdings

Endpoints: POST /api/ai/draft-document, /case-strategy, /similar-cases
All rate-limited (5 req/min) and permission-gated (PermAIExtraction).
YouPC database connection is optional (YOUPC_DATABASE_URL env var).

backend/internal/config/config.go

@@ -14,6 +14,7 @@ type Config struct {
 	SupabaseJWTSecret string
 	AnthropicAPIKey   string
 	FrontendOrigin    string
+	YouPCDatabaseURL  string // read-only connection to youpc.org Supabase for similar case finder
 }
 
 func Load() (*Config, error) {
@@ -26,6 +27,7 @@ func Load() (*Config, error) {
 		SupabaseJWTSecret: os.Getenv("SUPABASE_JWT_SECRET"),
 		AnthropicAPIKey:  os.Getenv("ANTHROPIC_API_KEY"),
 		FrontendOrigin:   getEnv("FRONTEND_ORIGIN", "https://kanzlai.msbls.de"),
+		YouPCDatabaseURL: os.Getenv("YOUPC_DATABASE_URL"),
 	}
 
 	if cfg.DatabaseURL == "" {