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/seed.go

@@ -0,0 +1,79 @@
+package models
+
+import (
+	"log"
+
+	"gorm.io/gorm"
+)
+
+// seedSiteSettings inserts the singleton site_settings row (id=1) if absent.
+// Text fields are left empty so templates fall back to i18n defaults until an
+// admin configures them.
+func seedSiteSettings(db *gorm.DB) {
+	var count int64
+	db.Model(&SiteSetting{}).Count(&count)
+	if count > 0 {
+		return
+	}
+	s := &SiteSetting{ID: 1}
+	if err := db.Create(s).Error; err != nil {
+		log.Printf("Warning: failed to seed site_settings: %v", err)
+	}
+}
+
+// seedUploadConfig inserts the singleton upload_configs row (id=1) if absent.
+func seedUploadConfig(db *gorm.DB) {
+	var count int64
+	db.Model(&UploadConfig{}).Count(&count)
+	if count > 0 {
+		return
+	}
+	c := &UploadConfig{
+		ID:             1,
+		Enabled:        true,
+		DefaultMaxSize: DefaultUploadMaxSize,
+		StorageDir:     "attachments",
+	}
+	if err := db.Create(c).Error; err != nil {
+		log.Printf("Warning: failed to seed upload_configs: %v", err)
+	}
+}
+
+// defaultUploadFileTypes is the set of commonly permitted attachment types
+// seeded on first run.
+var defaultUploadFileTypes = []UploadFileType{
+	// Images
+	{Extension: ".jpg", MimeType: "image/jpeg", Category: CategoryImage, Enabled: true, Sort: 1},
+	{Extension: ".jpeg", MimeType: "image/jpeg", Category: CategoryImage, Enabled: true, Sort: 2},
+	{Extension: ".png", MimeType: "image/png", Category: CategoryImage, Enabled: true, Sort: 3},
+	{Extension: ".gif", MimeType: "image/gif", Category: CategoryImage, Enabled: true, Sort: 4},
+	{Extension: ".webp", MimeType: "image/webp", Category: CategoryImage, Enabled: true, Sort: 5},
+	// Documents
+	{Extension: ".pdf", MimeType: "application/pdf", Category: CategoryDocument, Enabled: true, Sort: 10},
+	{Extension: ".doc", MimeType: "application/msword", Category: CategoryDocument, Enabled: true, Sort: 11},
+	{Extension: ".docx", MimeType: "application/vnd.openxmlformats-officedocument.wordprocessingml.document", Category: CategoryDocument, Enabled: true, Sort: 12},
+	{Extension: ".xls", MimeType: "application/vnd.ms-excel", Category: CategoryDocument, Enabled: true, Sort: 13},
+	{Extension: ".xlsx", MimeType: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", Category: CategoryDocument, Enabled: true, Sort: 14},
+	{Extension: ".ppt", MimeType: "application/vnd.ms-powerpoint", Category: CategoryDocument, Enabled: true, Sort: 15},
+	{Extension: ".pptx", MimeType: "application/vnd.openxmlformats-officedocument.presentationml.presentation", Category: CategoryDocument, Enabled: true, Sort: 16},
+	{Extension: ".txt", MimeType: "text/plain", Category: CategoryDocument, Enabled: true, Sort: 17},
+	// Archives
+	{Extension: ".zip", MimeType: "application/zip", Category: CategoryArchive, Enabled: true, Sort: 20},
+	{Extension: ".rar", MimeType: "application/vnd.rar", Category: CategoryArchive, Enabled: true, Sort: 21},
+	{Extension: ".7z", MimeType: "application/x-7z-compressed", Category: CategoryArchive, Enabled: true, Sort: 22},
+	// Video
+	{Extension: ".mp4", MimeType: "video/mp4", Category: CategoryVideo, Enabled: true, Sort: 30},
+	{Extension: ".avi", MimeType: "video/x-msvideo", Category: CategoryVideo, Enabled: true, Sort: 31},
+}
+
+// seedUploadFileTypes seeds the permitted file-type rows if the table is empty.
+func seedUploadFileTypes(db *gorm.DB) {
+	var count int64
+	db.Model(&UploadFileType{}).Count(&count)
+	if count > 0 {
+		return
+	}
+	if err := db.Create(&defaultUploadFileTypes).Error; err != nil {
+		log.Printf("Warning: failed to seed upload_file_types: %v", err)
+	}
+}