Reorganize repo: backend/, deploy/, docs/ layout + AGENTS.md

Restructure the monorepo into clear top-level directories:
- backend/: Go module root (cmd, internal, pkg, configs, migrations,
  docs/swagger, scripts, tests, go.mod, Makefile, .air.toml)
- deploy/: Docker (Dockerfile, docker-compose*), quickstart, fluentd
- docs/: project documentation + reports/ (moved from repo root)
- AGENTS.md: new AI coding-agent guide at repo root

Update all references to the new layout:
- Dockerfile: COPY backend/go.mod, COPY backend/ (context = repo root)
- docker-compose files: context ../.., dockerfile deploy/docker/Dockerfile,
  env_file ../../.env, volume mounts ../../backend:/app
- deploy/quickstart/compose.yaml: dockerfile deploy/docker/Dockerfile
- CI: working-directory: backend for go commands, file deploy/docker/Dockerfile,
  coverage path backend/coverage.out, health_check backend/scripts/
- backend/Makefile: docker target uses -f ../deploy/docker/Dockerfile ../
- README: architecture tree, quickstart, config paths updated

Move root stray scripts (rename_models.*, run_m11_tests.sh, verify_build.sh,
gorm_bool_main.go) to backend/scripts/legacy/. All moves via git mv to
preserve history. Build, vet, SQLite tests, and docker compose config verified.

backend/internal/model/widget_file_upload.go

@@ -0,0 +1,72 @@
+package model
+
+import (
+	"time"
+
+	"gorm.io/datatypes"
+)
+
+// WidgetFileUploadStatus constants for the staged upload process.
+type WidgetFileUploadStatus string
+
+const (
+	WidgetFileUploadStatusPending  WidgetFileUploadStatus = "pending"  // File uploaded, awaiting attachment to message
+	WidgetFileUploadStatusAttached WidgetFileUploadStatus = "attached" // File attached to a message
+	WidgetFileUploadStatusExpired  WidgetFileUploadStatus = "expired"  // Upload expired (not attached within timeout)
+)
+
+// WidgetFileUpload stores metadata about a file uploaded through the web widget.
+// Reference: Chatwoot widget SDK file upload — attachment processing pipeline
+// Widget file uploads go through a staged process:
+//   1. Visitor selects file in widget
+//   2. File is uploaded to storage via /widget/uploads endpoint
+//   3. Upload record is created with status "pending"
+//   4. When message is sent with attachment, status changes to "attached"
+type WidgetFileUpload struct {
+	Base
+	UploadUUID      string                 `gorm:"size:36;uniqueIndex;not null" json:"upload_uuid"`  // UUID for public-facing tracking (website_token staging)
+	WidgetToken     string                 `gorm:"size:255;index" json:"widget_token,omitempty"`     // Links to the widget session (widget_token auth)
+	InboxID         uint                   `gorm:"index;not null" json:"inbox_id"`                  // The inbox this upload belongs to
+	ContactID       uint                   `gorm:"index" json:"contact_id,omitempty"`               // Contact who uploaded (may be 0 for anonymous)
+	Status          WidgetFileUploadStatus `gorm:"size:20;default:'pending';index" json:"status"`  // "pending", "attached", "expired"
+
+	// === File metadata ===
+	OriginalName    string         `gorm:"size:255;not null" json:"original_name"`          // Original filename from user
+	FileType        string         `gorm:"size:50;not null" json:"file_type"`               // MIME type / category: "image", "audio", "video", "file"
+	MimeType        string         `gorm:"size:100" json:"mime_type,omitempty"`             // Full MIME type (e.g. "image/png")
+	FileSize        int64          `gorm:"not null" json:"file_size"`                        // File size in bytes
+	FileURL         string         `gorm:"size:512" json:"file_url,omitempty"`              // URL where file is stored (after upload)
+	ThumbURL        string         `gorm:"size:512" json:"thumb_url,omitempty"`             // Thumbnail URL (for images)
+
+	// === Dimensions (for images/videos) ===
+	Width           int            `json:"width,omitempty"`
+	Height          int            `json:"height,omitempty"`
+
+	// === Processing ===
+	Metadata        datatypes.JSON `gorm:"type:jsonb" json:"metadata,omitempty"`           // Additional metadata (checksums, etc)
+	MessageID       *uint          `gorm:"index" json:"message_id,omitempty"`               // Linked message (after attachment)
+	ExpiresAt       time.Time      `gorm:"index" json:"expires_at"`                         // Expiry timestamp for cleanup
+
+	Inbox Inbox `gorm:"foreignKey:InboxID" json:"inbox,omitempty"`
+}
+
+func (WidgetFileUpload) TableName() string { return "widget_file_uploads" }
+
+// WidgetUploadAllowedTypes defines the allowed MIME types for widget uploads.
+// Reference: Chatwoot widget SDK config — allowed file types restriction
+var WidgetUploadAllowedTypes = map[string][]string{
+	"image":  {"image/png", "image/jpeg", "image/gif", "image/webp", "image/svg+xml"},
+	"audio":  {"audio/mp3", "audio/ogg", "audio/wav", "audio/webm", "audio/mpeg"},
+	"video":  {"video/mp4", "video/webm", "video/ogg"},
+	"file":   {"application/pdf", "application/vnd.ms-excel", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
+		"application/msword", "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+		"text/plain", "text/csv"},
+}
+
+// WidgetUploadMaxSizeByType defines max file sizes per category.
+var WidgetUploadMaxSizeByType = map[string]int64{
+	"image": 5 * 1024 * 1024,   // 5MB for images
+	"audio": 10 * 1024 * 1024,  // 10MB for audio
+	"video": 15 * 1024 * 1024,  // 15MB for video
+	"file":  10 * 1024 * 1024,  // 10MB for documents
+}