feat: external video player flow and discord playback link improvements

.env.example

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+DISCORD_TOKEN=
+DISCORD_CLIENT_ID=
+DISCORD_GUILD_ID=
+DATABASE_URL=postgresql://postgres:postgres@localhost:5432/discord_multibot
+REDIS_URL=redis://localhost:6379/1
+LAVALINK_NODE_NAME=local
+LAVALINK_HOST=127.0.0.1
+LAVALINK_PORT=2333
+LAVALINK_PASSWORD=youshallnotpass
+LAVALINK_SECURE=false
+GEMINI_API_KEY=
+GEMINI_FLASH_MODEL=gemini-2.5-flash
+GEMINI_PRO_MODEL=gemini-2.5-pro
+DEEPL_API_KEY=
+DEEPL_CLI_BIN=deepl
+DEEPL_CLI_TIMEOUT_MS=45000
+GDS_DVIEWER_BASE_URL=http://127.0.0.1:9099/gds_dviewer/normal/explorer
+GDS_DVIEWER_API_KEY=
+GDS_DVIEWER_SOURCE_ID=0
+EXTERNAL_VIDEO_PLAYER_URL=
+IINA_LINK_ICON=🎬
+LOG_LEVEL=info