feat: add platform configuration tables and admin settings

Add four DB-backed configuration tables for site-wide settings:
site_settings (logo, top-left title, header/footer text with zh/en
variants), upload_configs (master switch, default size, storage dir),
upload_file_types (per-extension whitelist with per-type size limits),
and download_baseurls (multiple download sources with priority/default).

- Models, AutoMigrate, and first-run seed (18 common file types)
- Process-level config cache warmed at startup, refreshed on admin save
- SetUserContext injects cached site settings into templates
- base.html renders logo (local upload or external URL), title, header
  banner, and footer with i18n fallback
- Upload validator drives avatar uploads (form + AJAX) through the
  configured switch/type/size policy
- Admin pages for site/upload/download settings with i18n keys

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

models/upload_config.go

@@ -0,0 +1,74 @@
+package models
+
+import "time"
+
+// UploadCategory groups file types for the admin UI.
+const (
+	CategoryImage    = "image"
+	CategoryDocument = "document"
+	CategoryArchive  = "archive"
+	CategoryVideo    = "video"
+	CategoryOther    = "other"
+)
+
+// DefaultUploadMaxSize is the default per-file size limit (10 MiB), in bytes.
+const DefaultUploadMaxSize int64 = 10 * 1024 * 1024
+
+// UploadConfig holds the singleton (id=1) global attachment upload policy.
+type UploadConfig struct {
+	ID              uint      `gorm:"primarykey" json:"id"`
+	Enabled         bool      `gorm:"default:true" json:"enabled"`                 // master switch for attachment uploads
+	DefaultMaxSize  int64     `gorm:"default:10485760" json:"default_max_size"`    // bytes; overridden per type by UploadFileType.MaxSize
+	StorageDir      string    `gorm:"size:255;default:attachments" json:"storage_dir"` // sub-dir under cfg.Path
+	UpdatedBy       uint      `gorm:"index" json:"updated_by"`
+	UpdatedAt       time.Time `json:"updated_at"`
+}
+
+// TableName overrides the default GORM table name.
+func (UploadConfig) TableName() string {
+	return "upload_configs"
+}
+
+// UploadFileType describes one permitted attachment extension. Multiple rows.
+type UploadFileType struct {
+	ID        uint   `gorm:"primarykey" json:"id"`
+	Extension string `gorm:"size:32;uniqueIndex" json:"extension"` // with leading dot, e.g. ".pdf"
+	MimeType  string `gorm:"size:128" json:"mime_type"`             // associated MIME for validation
+	Category  string `gorm:"size:32;index" json:"category"`         // image/document/archive/video/other
+	MaxSize   int64  `gorm:"default:0" json:"max_size"`              // bytes; 0 means use UploadConfig.DefaultMaxSize
+	Enabled   bool   `gorm:"default:true" json:"enabled"`
+	Sort      int    `gorm:"default:0" json:"sort"`
+}
+
+// TableName overrides the default GORM table name.
+func (UploadFileType) TableName() string {
+	return "upload_file_types"
+}
+
+// EffectiveMaxSize returns the per-file size limit for this type, falling back
+// to the provided default when MaxSize is 0.
+func (t *UploadFileType) EffectiveMaxSize(def int64) int64 {
+	if t.MaxSize > 0 {
+		return t.MaxSize
+	}
+	return def
+}
+
+// DownloadBaseURL is one source base URL used to build attachment download
+// links. Multiple rows; the row marked IsDefault (or the highest-priority
+// enabled one) is used for generated links.
+type DownloadBaseURL struct {
+	ID        uint      `gorm:"primarykey" json:"id"`
+	Name      string    `gorm:"size:64" json:"name"`       // label, e.g. "主站" / "CDN"
+	BaseURL   string    `gorm:"size:512" json:"base_url"`  // e.g. https://cdn.example.com/uploads
+	Priority  int       `gorm:"default:0" json:"priority"` // lower = higher priority
+	IsDefault bool      `gorm:"default:false" json:"is_default"`
+	Enabled   bool      `gorm:"default:true" json:"enabled"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+// TableName overrides the default GORM table name.
+func (DownloadBaseURL) TableName() string {
+	return "download_baseurls"
+}