feat: Telegram Core API Integration - Webhook, Bot, Chat, Rate Limiting

[zp6]

Telegram Core API Integration (Closes #62)

Implements comprehensive Telegram Bot API integration covering webhook management, bot lifecycle, chat operations, rate limiting, and error handling.

Webhook Management

• SetWebhookPrism - Configure webhook URL with secret token verification, allowed updates filtering, max connections, and IP address pinning
• GetWebhookInfoPrism - Monitor webhook health, pending update count, last error diagnostics
• DeleteWebhookPrism - Remove webhook and switch to getUpdates polling mode

Bot Management

• GetBotInfoPrism - Bot identity verification via getMe, capabilities check
• GetUpdatesPrism - Long polling with offset tracking, timeout config, update type filtering

Chat Operations

• GetChatPrism - Chat metadata, member counts, pinned messages, photo info
• LeaveChatPrism - Leave groups, supergroups, or channels
• GetChatMemberCountPrism - Real-time member count tracking

Infrastructure

• RateLimiter (GenServer + ETS) - Sliding window rate limiting per chat type (global 30/s, group 20/min, private 1/s, channel 30/s) with automatic cleanup
• ErrorHandler - Exponential backoff with jitter, error classification (transient/fatal/rate_limited/conflict), max retry control, retry callbacks

Acceptance Criteria

• Bot authentication and info retrieval
• Webhook integration (set/get/delete)
• Rate limiting with per-chat-type limits
• Error handling with exponential backoff and retries
• Chat management operations