refactor(gateway): drop shared barrel, migrate to granular imports + shared error helpers

- delete src/shared/index.ts fat barrel; point 10 importers at the exact
  module they use (redis-channels, moderation-types, utils/pagination)
- message-capture/types.ts re-exports from shared/moderation-types directly
- shared/errors: add errorMessage() + isTransientStreamError() helpers,
  replacing the repeated err-message and transient-code checks
- drop unused imports flagged by biome

No behavior change. typecheck + lint + 138 tests green.

services/discord-gateway/src/modules/message-capture/messagesPagination.ts

@@ -2,8 +2,11 @@ import { and, desc, eq, inArray, type SQL, sql } from "drizzle-orm";
 import type { NodePgDatabase } from "drizzle-orm/node-postgres";
 import type * as schema from "../../shared/database/schema.js";
 import { messagesTable } from "../../shared/database/schema.js";
-import { buildCursorCondition, pageResult } from "../../shared/index.js";
 import { createChildLogger, type Logger } from "../../shared/logger/index.js";
+import {
+  buildCursorCondition,
+  pageResult,
+} from "../../shared/utils/pagination.js";
 import type {
   MessageQuery,
   MessageRecord,