feat: add document upload/download backend (Phase 2K)

- StorageClient for Supabase Storage REST API (upload, download, delete)
- DocumentService with CRUD operations + storage integration
- DocumentHandler with multipart form upload support (50MB limit)
- Routes: GET/POST /api/cases/{id}/documents, GET/DELETE /api/documents/{docId}
- file_path format: {tenant_id}/{case_id}/{uuid}_{filename}
- Case events logged on upload/delete
- Added SUPABASE_SERVICE_KEY to config for server-side storage access
- Fixed pre-existing duplicate writeJSON/writeError in appointments.go

backend/internal/config/config.go

@@ -6,12 +6,13 @@ import (
 )
 
 type Config struct {
-	Port             string
-	DatabaseURL      string
-	SupabaseURL      string
-	SupabaseAnonKey  string
+	Port              string
+	DatabaseURL       string
+	SupabaseURL       string
+	SupabaseAnonKey   string
+	SupabaseServiceKey string
 	SupabaseJWTSecret string
-	AnthropicAPIKey  string
+	AnthropicAPIKey   string
 }
 
 func Load() (*Config, error) {
@@ -19,7 +20,8 @@ func Load() (*Config, error) {
 		Port:             getEnv("PORT", "8080"),
 		DatabaseURL:      os.Getenv("DATABASE_URL"),
 		SupabaseURL:      os.Getenv("SUPABASE_URL"),
-		SupabaseAnonKey:  os.Getenv("SUPABASE_ANON_KEY"),
+		SupabaseAnonKey:   os.Getenv("SUPABASE_ANON_KEY"),
+		SupabaseServiceKey: os.Getenv("SUPABASE_SERVICE_KEY"),
 		SupabaseJWTSecret: os.Getenv("SUPABASE_JWT_SECRET"),
 		AnthropicAPIKey:  os.Getenv("ANTHROPIC_API_KEY"),
 	}