feat(api): streaming, transcoding, events, history and likes (M3)

- tracks carry a stream_url signed for the user for 12 h and the
  user's reaction; /stream checks the signature, expiry and that the
  user is not blocked, then hands the file to nginx by X-Accel-Redirect
  (or serves it with Range support when X_ACCEL=false)
- original by default; formats browsers can't play and explicit
  high/medium/low go through Opus transcoding, never upwards: a lossy
  source under the target bitrate stays original; transcodes are
  cached per file and size, deduplicated in flight, limited by
  TRANSCODE_CONCURRENCY and trimmed hourly by LRU to the size limit
- /events takes up to 500 events with client UUIDs, drops repeats and
  events older than 30 days, counts plays (≥ half or 4 min, or
  finished), early skips and history entries (≥ 30 s)
- history with clearing, likes and dislikes with cursors and totals,
  track reactions, playback state with last-write-wins, tracks batch

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/api/internal/platform/config/config.go

@@ -46,6 +46,9 @@ type API struct {
 	FFmpegPath string `env:"FFMPEG_PATH" envDefault:"ffmpeg"`
 	// XAccel — файлы отдаёт nginx по X-Accel-Redirect; false — для запуска api без nginx.
 	XAccel bool `env:"X_ACCEL" envDefault:"true"`
+	// TranscodeConcurrency — параллельных ffmpeg; 0 — половина ядер.
+	TranscodeConcurrency  int `env:"TRANSCODE_CONCURRENCY" envDefault:"0"`
+	TranscodeCacheLimitGB int `env:"TRANSCODE_CACHE_LIMIT_GB" envDefault:"50"`
 
 	MeiliURL       string `env:"MEILI_URL" envDefault:"http://meilisearch:7700"`
 	MeiliMasterKey string `env:"MEILI_MASTER_KEY"`