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/site_setting.go

@@ -0,0 +1,78 @@
+package models
+
+import "time"
+
+// SiteSetting holds the singleton (id=1) global display configuration:
+// logo, top-left title, header banner text, and footer text, each with
+// zh/en variants that fall back to i18n defaults when empty.
+type SiteSetting struct {
+	ID            uint      `gorm:"primarykey" json:"id"`
+	Logo          string    `gorm:"size:512" json:"logo"`            // local filename (served under /uploads/logos) OR a full URL
+	LogoTextZh    string    `gorm:"size:255" json:"logo_text_zh"`    // top-left title (zh)
+	LogoTextEn    string    `gorm:"size:255" json:"logo_text_en"`    // top-left title (en)
+	HeaderTextZh  string    `gorm:"size:512" json:"header_text_zh"`  // header banner text (zh)
+	HeaderTextEn  string    `gorm:"size:512" json:"header_text_en"`  // header banner text (en)
+	FooterTextZh  string    `gorm:"size:512" json:"footer_text_zh"`  // footer text (zh)
+	FooterTextEn  string    `gorm:"size:512" json:"footer_text_en"`  // footer text (en)
+	UpdatedBy     uint      `gorm:"index" json:"updated_by"`
+	UpdatedAt     time.Time `json:"updated_at"`
+}
+
+// TableName overrides the default GORM table name.
+func (SiteSetting) TableName() string {
+	return "site_settings"
+}
+
+// LogoIsURL reports whether the logo value is an external URL rather than a
+// local filename.
+func (s *SiteSetting) LogoIsURL() bool {
+	if s == nil || s.Logo == "" {
+		return false
+	}
+	return len(s.Logo) >= 4 && (s.Logo[:4] == "http")
+}
+
+// LogoText returns the title for the given language code, falling back to the
+// other language when the requested one is empty.
+func (s *SiteSetting) LogoText(lang string) string {
+	if lang == "zh" {
+		if s.LogoTextZh != "" {
+			return s.LogoTextZh
+		}
+		return s.LogoTextEn
+	}
+	if s.LogoTextEn != "" {
+		return s.LogoTextEn
+	}
+	return s.LogoTextZh
+}
+
+// HeaderText returns the header banner text for the given language code,
+// falling back to the other language when the requested one is empty.
+func (s *SiteSetting) HeaderText(lang string) string {
+	if lang == "zh" {
+		if s.HeaderTextZh != "" {
+			return s.HeaderTextZh
+		}
+		return s.HeaderTextEn
+	}
+	if s.HeaderTextEn != "" {
+		return s.HeaderTextEn
+	}
+	return s.HeaderTextZh
+}
+
+// FooterText returns the footer text for the given language code, falling
+// back to the other language when the requested one is empty.
+func (s *SiteSetting) FooterText(lang string) string {
+	if lang == "zh" {
+		if s.FooterTextZh != "" {
+			return s.FooterTextZh
+		}
+		return s.FooterTextEn
+	}
+	if s.FooterTextEn != "" {
+		return s.FooterTextEn
+	}
+	return s.FooterTextZh
+}