feat(ai-moderation): rich context + link media vision analysis

- Conversation context recency gates (GAP_MS/MAX_AGE_MS): drop stale
  messages before silence gaps; cold_start anchor + flow descriptor
  tells LLM whether conversation is ongoing or restarted
- [location] block: channel name, thread name, nsfw/age flags from
  captured metadata (thread names instead of bare IDs)
- Link media -> multimodal: text-batch URL fetches that resolve to
  images now run vision analysis (bounded 15s) and switch prompt to
  mixed mode; <web_content> gains og:title for page context
- pnpm-workspace.yaml: approve sharp build script (unblocks install)

services/discord-gateway/pnpm-workspace.yaml

@@ -3,6 +3,7 @@ allowBuilds:
   "@lng2004/node-datachannel": true
   esbuild: true
   node-av: true
+  sharp: true
   zeromq: true
 # pnpm 11 requires build-script approvals here (the legacy `pnpm` field in
 # package.json is ignored). Native voice deps need their postinstall build.