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/src/shared/config/index.ts

@@ -189,6 +189,17 @@ export const configSchema = z
       .int()
       .positive()
       .default(20),
+    // Recency gates for conversation context. A silence longer than GAP_MS
+    // between context messages = the conversation restarted (older messages
+    // dropped); MAX_AGE_MS caps how far back context is considered relevant.
+    AI_ANALYSIS_CONTEXT_GAP_MS: z.coerce
+      .number()
+      .positive()
+      .default(12 * 60 * 1000),
+    AI_ANALYSIS_CONTEXT_MAX_AGE_MS: z.coerce
+      .number()
+      .positive()
+      .default(45 * 60 * 1000),
     AI_ANALYSIS_PROCESSING_TIMEOUT_MS: z.coerce
       .number()
       .positive()