logging: re-level, redact user data at info, and key links by post id

P0 — level rework + redaction:
- info now carries only lifecycle, per-post business results (sent /
  forwarded / copied / template applied), admin actions and anomalies
  (upload fallback, retry enqueue; dead-letter stays error).
- Per-request detail moved to debug: message/command logging, URL
  extraction, fetching/fetched, link-cache hits, media-group batch sends,
  queue processing (enqueue/processing/completed/rescheduled), photo
  processing (downscale/transcode), inline queries, sensitive-tweet note.
- Full user-submitted URLs and message text now appear only at debug; at
  info and above links are printed via the normalized cache key.

P1 — request correlation:
- handlers::log_key() maps a URL to its normalized post key
  (twitter:<id> / pixiv:<id> / bsky:<handle>/<rkey>). The whole lifecycle
  of one link (fetch -> send -> cache -> fallback) now logs [key=...], so
  multi-worker logs can be correlated by grepping the key.

Convention documented in AGENTS.md.

crates/xmedia-bot/src/queue.rs

@@ -188,7 +188,7 @@ impl PersistentTaskQueue {
             self.counter.fetch_add(1, Ordering::Relaxed)
         );
         let payload = payload.to_string();
-        log::info!("enqueued {id} (run_after {run_after:.1})");
+        log::debug!("enqueued {id} (run_after {run_after:.1})");
         self.pool.with_conn(move |conn| {
             conn.execute(
                 "INSERT OR REPLACE INTO tasks (id, payload, run_after, attempts, status, locked_until, created_at) \
@@ -339,10 +339,10 @@ impl QueueWorker {
                 return;
             }
         };
-        log::info!("processing {} (attempt {})", row.id, row.attempts + 1);
+        log::debug!("processing {} (attempt {})", row.id, row.attempts + 1);
         match (self.handler)(payload).await {
             Ok(()) => {
-                log::info!("task {} completed", row.id);
+                log::debug!("task {} completed", row.id);
                 self.delete_row(&row.id).await;
             }
             Err(QueueError::Retryable {
@@ -356,7 +356,7 @@ impl QueueWorker {
                     (self.dead_letter)(payload, message).await;
                 } else {
                     let delay = scaled_retry_delay(delay_seconds, row.attempts);
-                    log::info!(
+                    log::debug!(
                         "task {} rescheduled in {delay:.1}s (attempt {})",
                         row.id,
                         row.attempts + 1