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.

crates/xmedia-bot/src/state.rs

@@ -73,7 +73,7 @@ impl ChatStore {
             })
             .await
             .unwrap_or_else(|e| {
-                log::error!("chat_state read failed: {e}");
+                log::warn!("chat_state read failed: {e}");
                 None
             })
             .unwrap_or_default();
@@ -98,7 +98,7 @@ impl ChatStore {
             })
             .await;
         if let Err(e) = result {
-            log::error!("chat_state write failed: {e}");
+            log::warn!("chat_state write failed: {e}");
         }
     }