feat: port missing MediaStation features (DLNA, STRM, files, dup, sched, api-configs, emby, storage)

Audit-driven port from the original Python MediaStation. Eight major
subsystems that were absent from the Go rewrite are now in place,
each with its own service, handler, frontend page and smoke-test
assertions.

Backend services
  - service/crypto.go: AES-256-GCM encrypt/decrypt for at-rest secrets
    keyed off the JWT secret. Legacy plaintext rows pass through
    unchanged for smooth upgrades. Unit-tested.
  - service/api_config.go: third-party provider config (TMDb, Bangumi,
    TheTVDB, Fanart, Douban, OpenAI). Seeds defaults on first run.
    Encrypts api_key on write, returns masked 'abc1****wxyz' projection.
  - service/duplicate.go: sparse-sample MD5 (head + middle + tail, 1MiB
    each, plus file-size suffix) duplicate finder. Picks 'best' primary
    (matched > size > id) and marks others is_duplicate=true.
  - service/filemanager.go: server-side allow-listed file browser used
    by the library-path picker. Strict path-traversal protection.
  - service/dlna.go: real SSDP M-SEARCH discovery + AVTransport
    SetAVTransportURI/Play SOAP cast. 30 s discovery cache.
  - service/scheduler.go: 3 recurring background jobs (library_scan
    60min, transcode_cleanup 24h, recycle_purge 24h with 30-day
    cutoff). Status + run-now endpoints.
  - service/cache_cleanup.go: walkAndPrune helper used by scheduler.
  - service/storage.go: DB-only disk-usage breakdown by library and by
    container format.
  - service/emby_compat.go: read-only Emby/Jellyfin shim
    (System/Info, Users, Users/x/Views, Items, PlaybackInfo) so Infuse
    / VidHub / Kodi can browse MediaStationGo libraries.

Model updates
  - Media: new strm_url (302 redirect target), file_hash, is_duplicate,
    duplicate_of fields.
  - APIConfig: new table for encrypted provider secrets.
  - AutoMigrate registers APIConfig.

Stream layer
  - StreamService.ServeFile now redirects 302 to strm_url when set so
    WebDAV / Alist / S3 / HTTP direct links work transparently.

Handlers + routes
  - Authed: GET /files, GET /storage, GET /dlna/devices, POST /dlna/cast,
    PUT/DELETE /media/:id/strm, POST /strm/import,
    POST /duplicates/{scan,unmark}.
  - Admin: GET/PUT/DELETE /admin/api-configs/:provider,
    GET /admin/scheduler, POST /admin/scheduler/:name/run.
  - New /emby/* group: System/Info, Users, Users/:userId/Views,
    Users/:userId/Items, Items/:id/PlaybackInfo (auth-required).

Frontend pages (lazy-loaded, 7 new chunks)
  - DlnaPage: device list + media picker + cast button.
  - FileManagerPage: root selector + breadcrumb + sortable listing.
  - APIConfigsPage: per-provider card with masked-key editor.
  - StoragePage: usage tiles + per-library bars + per-container grid.
  - DuplicatesPage: scan form + grouped report with primary highlight.
  - SchedulerPage: live job table with run-now button (5s refresh).
  - Sidebar reorganised: 自动化 group adds DLNA, 管理 group adds
    存储 / 文件浏览 / 重复文件 / 定时任务 / API 配置.

Smoke test additions (all admin-only)
  - api-configs seeded with 6 providers
  - api-config encrypted in db (sqlite3 enc:v1: prefix check)
  - storage breakdown
  - file browser lists library root + rejects /etc (path traversal)
  - dlna devices endpoint
  - scheduler exposes 3 jobs + run library_scan
  - emby /System/Info + /Users/{x}/Views
  - strm set + stream 302 + strm clear
  - duplicate scan

Verified: go build, go vet, go test (incl. new TestCrypto* suite + the
existing TestParseEpisode/TestCleanQuery/TestSrtToVTT/TestStripASSTags/
TestBuildFFmpegArgs); tsc -b && vite build emits 28 route chunks plus
the deferred hls chunk; main bundle 253 KB / 85 KB gzipped; smoke test
PASS=42 / FAIL=0.

internal/model/model.go

@@ -79,6 +79,41 @@ type Media struct {
 	TMDbID       int     `json:"tmdb_id"`
 	BangumiID    int     `json:"bangumi_id"`
 	NSFW         bool    `gorm:"default:false" json:"nsfw"`
+
+	// STRMURL is the indirection target for .strm files: when present the
+	// stream handler redirects to it instead of opening the local file.
+	// Used to expose WebDAV / Alist / S3 / HTTP direct links as media items.
+	STRMURL string `gorm:"size:2048" json:"strm_url,omitempty"`
+
+	// FileHash is a sparse-sample MD5 used for duplicate detection.
+	// Computed on-demand by the duplicate finder; format: "<hex>-<size>".
+	FileHash string `gorm:"index;size:64" json:"file_hash,omitempty"`
+
+	// IsDuplicate flags this media as a duplicate of another media row.
+	IsDuplicate bool   `gorm:"default:false" json:"is_duplicate"`
+	DuplicateOf string `gorm:"size:36" json:"duplicate_of,omitempty"`
+}
+
+// APIConfig stores third-party data-source configuration. The api_key
+// column is encrypted with AES-GCM (see internal/service/crypto.go) so an
+// SQLite leak does not expose third-party credentials.
+//
+// Provider values mirror the original Python project:
+//
+//	tmdb        — themoviedb.org
+//	bangumi     — bgm.tv
+//	thetvdb     — thetvdb.com
+//	fanart      — fanart.tv
+//	douban      — douban.com (cookie)
+//	openai      — OpenAI / DeepSeek / Qwen / Ollama (compatible)
+type APIConfig struct {
+	Base
+	Provider    string `gorm:"uniqueIndex;size:32;not null" json:"provider"`
+	APIKey      string `gorm:"type:text" json:"-"`              // ciphertext (never serialised)
+	BaseURL     string `gorm:"size:512" json:"base_url,omitempty"`
+	Extra       string `gorm:"type:text" json:"extra,omitempty"` // free-form JSON
+	Enabled     bool   `gorm:"default:true" json:"enabled"`
+	Description string `gorm:"size:255" json:"description,omitempty"`
 }
 
 // Series groups episodes that belong to the same show.
@@ -185,5 +220,6 @@ func AllModels() []interface{} {
 		&Subscription{},
 		&Setting{},
 		&AccessLog{},
+		&APIConfig{},
 	}
 }