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/x-media/src/site/mod.rs

@@ -329,8 +329,10 @@ pub async fn fetch(url: &str) -> Result<Option<Fetched>, FetchError> {
     for attempt in 0..3u32 {
         match fetch_once(url).await {
             Ok(Some(fetched)) => {
-                log::info!(
-                    "fetched {url}: site {} returned {} media",
+                // Per-request detail: debug only, keyed by the post id.
+                log::debug!(
+                    "fetched [key={}]: site {} returned {} media",
+                    cache_key(url).unwrap_or_else(|| "?".into()),
                     fetched.site_name(),
                     fetched.media.len()
                 );

crates/x-media/src/site/twitter/interface.rs

@@ -35,7 +35,7 @@ pub async fn fetch_from_url(url: &str) -> Result<Fetched, FetchError> {
                     }
                 }
             } else {
-                log::info!("tweet {id} is sensitive; set TWITTER_AUTH_TOKEN to fetch NSFW media");
+                log::debug!("tweet {id} is sensitive; set TWITTER_AUTH_TOKEN to fetch NSFW media");
                 Ok(empty_fetched(url))
             }
         }