Support authentication for URL fetching

[SashankBhamidi]

Description

Many calendar services require authentication to access calendar URLs. Add support for authenticated URL fetching.

Use Cases

1. Google Calendar - OAuth tokens or service account credentials
2. Office 365 / Outlook - Microsoft authentication
3. CalDAV servers - Basic auth (username/password)
4. Custom APIs - API keys, bearer tokens

Proposed Implementation

Add optional authentication parameters to the /fetch endpoint:

class FetchRequest(BaseModel):
    url: str
    auth_type: Optional[str] = None  # "basic", "bearer", "oauth"
    credentials: Optional[dict] = None

Frontend Changes

Add authentication options to the Fetch URL tab:

• Auth type dropdown (None, Basic, Bearer Token, etc.)
• Conditional credential inputs based on auth type
• Secure handling (don't log credentials)

Security Considerations

• Credentials should NOT be logged
• Consider time-limited tokens over passwords
• Rate limiting per credential (prevent abuse)
• Document security best practices

References

• Issue Frontend - URL fetching #6 - nicco's comment about Open Web Calendar authentication
• Open Web Calendar authentication docs

Related Issues

• Frontend - URL fetching #6 - Frontend URL fetching (basic implementation complete)
• Enhance SSRF protection against DNS rebinding attacks #70 - SSRF protection enhancements