refactor: remove YouPCDatabaseURL, use same DB connection for case finder

Now that KanzlAI is on the youpc.org Supabase instance, the separate
YouPCDatabaseURL connection is unnecessary. The main database connection
can query mlex.* tables directly since they're on the same Postgres.

- Remove YouPCDatabaseURL from config
- Remove separate sqlx.Connect block in main.go
- Pass main database handle as youpcDB parameter to router
- Update CLAUDE.md: mgmt schema in youpc.org (was kanzlai in flexsiebels)

backend/internal/config/config.go

@@ -14,7 +14,6 @@ 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) {
@@ -27,7 +26,6 @@ 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 == "" {