feat(core): implement data retention, metrics, and enhanced media handling

This commit introduces several significant improvements across the backend and gateway services:

- **Data Retention**: Added an automated cleanup scheduler in `discord-gateway` to prune expired messages, attachments, and voice recordings based on configurable retention policies.
- **Observability**: Integrated `prom-client` in the `backend` service to expose Prometheus metrics via `/api/metrics` and added default Node.js runtime metrics.
- **Media Handling**: Enhanced `MediaHandler` in `discord-gateway` to support media URL resolution and improved playback status tracking.
- **API & Config**: Expanded the configuration endpoint to expose more system settings and reorganized `.env.example` for better readability.
- **Refactoring & Cleanup**:
    - Removed unused `better-sqlite3` dependency.
    - Refactored voice channel routing.
    - Improved error handling and testing coverage with comprehensive unit tests for shared utilities and error classes.
- **Documentation**: Added `MEMORY.md` for project context.

MEMORY.md

@@ -0,0 +1,92 @@
+# Durable Memory Wiki
+
+Consolidated knowledge and long-term facts.
+
+## Core Learnings
+
+- **Topic:** session
+  **Context:** Failure observation: session
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** biome check diagnosticlevelerror
+  **Context:** Failure observation: Exit code 1
+$ biome check --diagnostic-level=error .
+src/modules/voice-recording/recorder/segment.ts:1:1 assist/source/organizeImports  FIXABLE  ━━━━━━━━━━
+
+  × Sort these imports
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** typecheck scope workspace
+  **Context:** Failure observation: Exit code 2
+$ pnpm -r run typecheck
+Scope: 6 of 7 workspace projects
+packages/shared typecheck$ tsc --noEmit
+packages/shared typecheck: Done
+services/backend typecheck$ tsc --noEmit
+services/discord-g
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** eisdir illegal operation
+  **Context:** Failure observation: EISDIR: illegal operation on a directory, read '/mnt/code/bete/packages/shared/src/types/'
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** exist current working
+  **Context:** Failure observation: File does not exist. Note: your current working directory is /mnt/code/bete.
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** error 32603 pattern
+  **Context:** Failure observation: MCP error -32603: pattern must be a non-empty string.
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** errpnpmnoscript missing script
+  **Context:** Failure observation: Exit code 2
+[ERR_PNPM_NO_SCRIPT] Missing script: build:shared
+
+Command "build:shared" not found. Did you mean "pnpm run build:backend"?
+$ tsc
+$ pnpm --filter './services/backend' run build
+$ tsc
+$ pnp
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** projects matched filters
+  **Context:** Failure observation: Exit code 1
+No projects matched the filters "vendor/*" in "/mnt/code/bete"
+Scope: 6 of 7 workspace projects
+vendor/discord.js-selfbot-v13 test$ npm run lint && npm run test:typescript && npm run docs:
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** scope workspace projects
+  **Context:** Failure observation: Exit code 1
+$ pnpm -r run test
+Scope: 6 of 7 workspace projects
+vendor/discord.js-selfbot-v13 test$ npm run lint && npm run test:typescript && npm run docs:test
+vendor/discord.js-selfbot-v13 test: > d
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** error 32603 include
+  **Context:** Failure observation: MCP error -32603: include must be an array of strings.
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** nodeinternalmodulescjsloader1522 throw error
+  **Context:** Failure observation: Exit code 1
+node:internal/modules/cjs/loader:1522
+  throw err;
+  ^
+
+Error: Cannot find module 'ioredis'
+Require stack:
+- /mnt/code/bete/[eval]
+    at Module._resolveFilename (node:internal/modules/cjs
+  *Promoted on:* 2026-06-10T03:02:46.068Z
+
+- **Topic:** biome check diagnosticlevelerror
+  **Context:** Failure observation: Exit code 1
+$ biome check --diagnostic-level=error src/
+src/features/messages/index.tsx:4:1 assist/source/organizeImports  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━�
+  *Promoted on:* 2026-06-10T09:16:35.182Z
+
+- **Topic:** eval1 matches found
+  **Context:** Failure observation: Exit code 1
+(eval):1: no matches found: tsconfig*.json
+  *Promoted on:* 2026-06-10T09:49:12.792Z