feat: align fmcd behavior with fedimint-client 0.8

[okjodom]

The fmcd API now provides:

• Automatic invoice monitoring - Using fedimint's native subscribe_ln_receive
• Real-time streaming updates - SSE endpoints for live invoice status
• Bulk status queries - Efficient multi-invoice management
• Full fedimint 0.8 alignment - Native behavior matching the underlying client

[Copilot]

Pull Request Overview

This PR modernizes fmcd to align with fedimint-client 0.8 behavior, introducing automatic invoice monitoring, real-time streaming, and bulk operations while removing deprecated external pubkey APIs.

• Replaces manual invoice polling with automatic fedimint native monitoring using subscribe_ln_receive
• Adds real-time SSE endpoints for live invoice status streaming and bulk operations
• Removes deprecated external pubkey tweaked APIs and await methods

Reviewed Changes

Copilot reviewed 23 out of 25 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/webhooks/invoice.rs	New webhook notification system for invoice events with exponential backoff
src/router/handlers/ln/invoice.rs	Enhanced invoice creation with automatic monitoring and unified status tracking
src/router/handlers/ln/status.rs	New unified status endpoints supporting bulk queries and operation ID lookup
src/router/handlers/ln/stream.rs	SSE streaming endpoints for real-time invoice monitoring and global events
src/router/ws.rs	Updated WebSocket routing to use new status endpoints and correlation tracking
src/main.rs	Updated REST API routes to align with fedimint 0.8 endpoint structure

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