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/app/retention.ts

@@ -1,8 +1,5 @@
 import { inArray, lt } from "drizzle-orm";
-import type {
-  NodePgDatabase,
-  NodePgQueryResultHKT,
-} from "drizzle-orm/node-postgres";
+import type { NodePgDatabase } from "drizzle-orm/node-postgres";
 import { createChildLogger } from "@/shared/logger/index";
 import { config } from "../shared/config/index.js";
 import { getDatabase } from "../shared/database/drizzle.js";