feat(log): survive a bare deployment and name what each line is about

P0 (foundation) + P1 (diagnostic depth) of the logging plan:

- main.rs initializes the timed builder with a default filter of
  `info,hyper_util=warn,reqwest=warn`. Without RUST_LOG nothing was logged at
  all (env_logger falls back to `error`), so `docker run --env-file .env` was
  silent, and the plain `init` had no timestamps.
- info-and-above lines stop printing user URLs (fetch/send failures, inline
  fetch, bsky's remux warnings). The full URL, the message text and the inline
  query move to `trace`, so a `debug` log can be handed to someone else.
- Lifecycle lines name the chat and the post: sent/failed/queued plus the
  total `ms`, the edit prompt, the channel forward, and every queue line
  (`chat=` + `[key=…]` + per-attempt `ms`, dead-letters included).
- Queue work is visible: `x-media`'s fetch line carries its duration (ugoira
  encode and HLS remux included), and the 300s sweep reports the pending count
  and how overdue the oldest task is — only when the queue is non-empty.
- URL workers are supervised like the queue workers: a panicking worker used
  to die silently and shrink the pool for the rest of the process.
- Degradations that still serve the user (cache/state write or read failures,
  a failed chat action) are `warn`, not `error`.

Verified against the scripted fake-API harness: unset RUST_LOG logs info with
timestamps, `debug` carries no user URL, `trace` does, a cache-hit send logs
`chat=111 in 5ms`, a failing send queues and dead-letters with chat+key, and
the sweep reports the pending retry.

README.md

@@ -93,7 +93,7 @@ Telegram 只接受 443/80/88/8443 端口。
 | `LINK_CACHE_TTL_SECONDS` | 链接结果缓存过期秒数，默认 604800（7 天） |
 | `CAPTION_QUOTE_TEXT_CHARS` | 正文（`{title}` + `{content}` 合计）达到该长度（字符）时，caption 的**正文部分**用可折叠引用块包裹，默认 200；`0` 关闭 |
 | `DATA_DIR` | 数据目录（SQLite 数据库 `task_queue.db` 所在目录），默认 `data`（相对工作目录，会自动创建） |
-| `RUST_LOG` | 日志级别 |
+| `RUST_LOG` | 日志级别，默认 `info,hyper_util=warn,reqwest=warn`（未设置也**不会**哑掉）。排障配方：`info,xmedia_bot=debug,x_media=debug`（应用细节，无依赖噪音）/ `debug,hyper_util=off`（全量）/ `trace`（额外打印完整链接与消息原文，**含用户数据**） |
 | `TELOXIDE_PROXY` | HTTP 代理（如 `http://127.0.0.1:10808`）；同时作用于 Telegram Bot API 与站点抓取请求，网络受限环境（如 GFW）必需 |
 | `LOCAL_USER_ID` | 容器内运行用户 UID，默认 9001 |
 | `VIRTUAL_HOST` | 对外域名或 IP，nginx-proxy 按此路由 |