feat: Add Telegram bot ingestion and user linking flow

[klinki]

Motivation

• Add official Telegram ingestion (long-polling bot) to allow users to forward private-chat text/links into the existing capture pipeline.
• Provide a simple, secure linking flow so a shared bot can deliver messages to the correct Sentinel user.
• Reuse the existing capture normalization and processing pipeline rather than inventing a new ingestion path.

Description

• Add domain entities TelegramChatLink, TelegramLinkCode, and TelegramIngestionState and register them in ApplicationDbContext to persist link bindings, link codes, and poll cursors.
• Implement ITelegramIntegrationService and TelegramIntegrationService to issue/consume short-lived link codes, filter private text updates, normalize messages into CaptureRequestDto (ContentType = Note, metadata/tags/labels) and enqueue processing.
• Add authenticated API endpoints under api/v1/integrations/telegram for GET /status, POST /link-code, and DELETE /link to manage linking and show pending codes.
• Wire configuration and DI: add TelegramIntegrationOptions to appsettings, register options, register the service in application DI, and add a worker hosted service TelegramPollingHostedService to run long-poll cycles in the worker.
• Frontend: add TelegramLinkStatus and TelegramLinkCodeResponse types and methods to AuthService, extend the Settings UI with a Telegram section to request link codes, display status/countdown, and unlink, and update the settings component test stubs to include the new calls.

Testing

• Ran the settings component unit spec with Vitest: npm run test -- --watch=false --include src/app/features/settings/settings.component.spec.ts and the spec passed ✅.
• Attempted full frontend build npm run build, but it failed in this environment due to external Google Fonts inlining returning HTTP 403 (environmental/network constraint) ⚠️.
• Attempted dotnet build but the dotnet CLI is not available in this environment, so backend compilation could not be executed here ⚠️.

---

Codex Task