fix(ai-moderation): resolve race conditions and implement processing state

Introduces a `processing` state to the AI analysis lifecycle to prevent
duplicate processing of the same messages.

- Implements row-level locking using `FOR UPDATE SKIP LOCKED` in
  `messageStore.ts` to ensure atomic message acquisition.
- Adds a `processing` status to the `AIStatus` type and database schema.
- Fixes a TOCTOU race condition in `aiAnalyzer.ts` by synchronizing
  the conversation processing lock before async database operations.
- Implements `revertStuckProcessingMessages` to recover messages stuck
  in the `processing` state due to worker crashes or timeouts.
- Updates `processBatch` and scheduling logic to correctly manage and
  release conversation-level locks.

services/discord-gateway/src/shared/database/schema.ts

@@ -62,7 +62,7 @@ export const pgMessagesTable = pgTable(
       .default("text"),
     metadata: pgText("metadata"),
     ai_status: pgText("ai_status", {
-      enum: ["pending", "clean", "warn", "flagged", "error"],
+      enum: ["pending", "processing", "clean", "warn", "flagged", "error"],
     })
       .notNull()
       .default("pending"),