feat: site management + cross-site torrent search

Port the complete site management system from the original MediaStation:

Model:
  - model/model.go: Added Site table with fields for name, base_url,
    site_type (nexusphp/gazelle/unit3d/mteam/custom_rss), auth_type
    (cookie/api_key/authorization), cookie, api_key, auth_header,
    user_agent, rss_url, timeout, priority, use_proxy, enabled,
    login_status, downloader. Registered in AllModels().

Backend services:
  - service/site.go: SiteService with CRUD (Create/List/FindByID/Update/
    Delete), TestConnection (validates credentials via HTTP, updates
    login_status in DB), and Search (cross-site fan-out that queries
    every enabled site adapter and returns merged results sorted by
    seeders descending).
  - service/site_adapter.go: SiteAdapter interface + NewSiteAdapter
    factory + 5 implementations:
    * nexusPhpAdapter — regex HTML scraping of torrents.php
    * gazelleAdapter — JSON API /ajax.php?action=browse
    * unit3dAdapter — REST API /api/torrents/filter
    * mteamAdapter — M-Team v3 POST /api/torrent/search with x-api-key
    * rssAdapter — XML RSS/Atom feed parsing with keyword filter

Handler + routes:
  - handler/sites.go: listSites, getSite, createSite, updateSite,
    deleteSite, testSite, siteSearch handlers.
  - handler/handler.go: 7 new routes under authed group:
    GET/POST /sites, GET/PUT/DELETE /sites/:id,
    POST /sites/:id/test, GET /sites/search.

Frontend:
  - api/sites.ts: typed interfaces (Site, SiteSearchResult,
    CreateSiteInput) and sitesAPI helper.
  - pages/SitesPage.tsx: full CRUD UI with add-site form (site_type +
    auth_type selectors, cookie/api_key/rss inputs), connection test
    button with live status indicator, delete action.
  - pages/SiteSearchPage.tsx: cross-site search with keyword input,
    merged result table (site/title/size/seeders/leechers/free),
    one-click download-to-qBittorrent button.
  - App.tsx: lazy-loaded routes /sites and /site-search.
  - Layout.tsx: sidebar links 站点管理 + 站点搜索 under 自动化 group.

Verified: go build + go vet + go test pass; tsc -b + vite build passes
(32 lazy chunks, main bundle 255 KB / 85 KB gzipped).

internal/model/model.go

@@ -205,6 +205,30 @@ type AccessLog struct {
 	Detail string `gorm:"type:text" json:"detail"`
 }
 
+// Site stores a PT/BT tracker site configuration used by the subscription
+// and cross-site search system. Mirrors the original MediaStation sites table.
+//
+// Supported site types: nexusphp / gazelle / unit3d / mteam / custom_rss
+// Supported auth types: cookie / api_key / authorization
+type Site struct {
+	Base
+	Name       string `gorm:"size:128;not null" json:"name"`
+	BaseURL    string `gorm:"size:512;not null" json:"base_url"`
+	SiteType   string `gorm:"size:32;default:nexusphp" json:"site_type"`
+	AuthType   string `gorm:"size:32;default:cookie" json:"auth_type"`
+	Cookie     string `gorm:"type:text" json:"cookie,omitempty"`
+	APIKey     string `gorm:"size:512" json:"api_key,omitempty"`
+	AuthHeader string `gorm:"size:512" json:"auth_header,omitempty"`
+	UserAgent  string `gorm:"size:512" json:"user_agent,omitempty"`
+	RSSURL     string `gorm:"size:1024" json:"rss_url,omitempty"`
+	Timeout    int    `gorm:"default:15" json:"timeout"`
+	Priority   int    `gorm:"default:50" json:"priority"`
+	UseProxy   bool   `gorm:"default:false" json:"use_proxy"`
+	Enabled    bool   `gorm:"default:true" json:"enabled"`
+	LoginStatus string `gorm:"size:20;default:unknown" json:"login_status"`
+	Downloader  string `gorm:"size:50" json:"downloader,omitempty"`
+}
+
 // AllModels returns the slice consumed by gorm.AutoMigrate.
 func AllModels() []interface{} {
 	return []interface{}{
@@ -219,6 +243,7 @@ func AllModels() []interface{} {
 		&DownloadTask{},
 		&Subscription{},
 		&Setting{},
+		&Site{},
 		&AccessLog{},
 		&APIConfig{},
 	}