feat: implement exponential backoff for paid_invoices_stream (#778)

* feat: implement exponential backoff for paid_invoices_stream across all Lightning backends

- Add exponential backoff retry logic to CLN REST, LND REST, and LND gRPC backends
- Start with 1 second delay, exponentially increase up to 5 minutes maximum
- Reset delay to 1 second on successful reconnection
- Improve error logging to include retry delay information
- Replace fixed delays with adaptive backoff to handle network issues gracefully
- Prevents system overload during persistent connection problems

Resolves issues with rapid reconnection attempts that could overwhelm Lightning nodes during network instability.

* remove unused import

* feat: extend exponential backoff to all remaining backends and invoice listener

- Implement exponential backoff in LNbits paid_invoices_stream for both SSE and WebSocket modes
- Add exponential backoff guidance comments to Blink and Strike backends (not implemented)
- Apply exponential backoff to invoice_listener in tasks.py that calls paid_invoices_stream
- Ensure consistent retry behavior across all Lightning backend integrations
- Improve system resilience during network interruptions and backend failures

All backends and the invoice listener now use the same exponential backoff strategy:
- Start with 1 second delay, exponentially increase up to 5 minutes maximum
- Reset delay to 1 second on successful reconnection
- Enhanced error logging with retry delay information

* blink + strike remove comments

* remove hardcoded values in favor of settings

* immediate first retry

cashu/core/settings.py

@@ -71,6 +71,9 @@ class MintSettings(CashuSettings):
         description="Interval (in seconds) for running regular tasks like the invoice checker.",
     )
 
+    mint_retry_exponential_backoff_base_delay: int = Field(default=1)
+    mint_retry_exponential_backoff_max_delay: int = Field(default=10)
+
 
 class MintWatchdogSettings(MintSettings):
     mint_watchdog_enabled: bool = Field(