feat(upload): add file upload API with S3 + CloudFront signed cookies

Add POST /api/upload-file endpoint that uploads files to S3 and returns
CDN URLs protected by CloudFront signed cookies (same pattern as Linear).

Infrastructure:
- Two private S3 buckets (static.multica.ai, static-staging.multica.ai)
- Two CloudFront distributions with OAC and Trusted Key Groups
- ACM wildcard cert in us-east-1, DNS records in Route 53
- RSA signing key stored in AWS Secrets Manager

Backend:
- S3 storage service with CloudFront CDN domain support
- CloudFront signed cookie generation (RSA-SHA1)
- Private key loaded from Secrets Manager (env var fallback for local dev)
- Cookies set on login (VerifyCode) with 72h expiry matching JWT
- Upload handler: multipart form → S3 → CloudFront URL response

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/internal/handler/handler.go

@@ -12,10 +12,12 @@ import (
 	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/jackc/pgx/v5/pgtype"
 	db "github.com/multica-ai/multica/server/pkg/db/generated"
+	"github.com/multica-ai/multica/server/internal/auth"
 	"github.com/multica-ai/multica/server/internal/events"
 	"github.com/multica-ai/multica/server/internal/middleware"
 	"github.com/multica-ai/multica/server/internal/realtime"
 	"github.com/multica-ai/multica/server/internal/service"
+	"github.com/multica-ai/multica/server/internal/storage"
 	"github.com/multica-ai/multica/server/internal/util"
 )
 
@@ -38,9 +40,11 @@ type Handler struct {
 	TaskService  *service.TaskService
 	EmailService *service.EmailService
 	PingStore    *PingStore
+	Storage      *storage.S3Storage
+	CFSigner     *auth.CloudFrontSigner
 }
 
-func New(queries *db.Queries, txStarter txStarter, hub *realtime.Hub, bus *events.Bus, emailService *service.EmailService) *Handler {
+func New(queries *db.Queries, txStarter txStarter, hub *realtime.Hub, bus *events.Bus, emailService *service.EmailService, s3 *storage.S3Storage, cfSigner *auth.CloudFrontSigner) *Handler {
 	var executor dbExecutor
 	if candidate, ok := txStarter.(dbExecutor); ok {
 		executor = candidate
@@ -55,6 +59,8 @@ func New(queries *db.Queries, txStarter txStarter, hub *realtime.Hub, bus *event
 		TaskService:  service.NewTaskService(queries, hub, bus),
 		EmailService: emailService,
 		PingStore:    NewPingStore(),
+		Storage:      s3,
+		CFSigner:     cfSigner,
 	}
 }