Implement iCal API endpoint for event calendar subscriptions

[Copilot]

This PR implements the ical_subscription web endpoint that serves iCal formatted calendar data for user events, as requested in the issue.

Changes Made

Core Implementation

• Replaced placeholder code in backend/src/web/ical_subscription.rs with a complete iCal generation implementation
• Changed parameter from _user_id to user_id and properly use it to filter events by author
• Added icalendar crate dependency (v0.16.7) to Cargo.toml for RFC 5545 compliant calendar generation

API Behavior

• Endpoint: GET /calendar.ics?user_id=<id>
• Returns: Proper iCal format with text/calendar content type
• Anonymous access: Uses get_events RPC without user authentication as specified
• Event filtering: Fetches events where the specified user is the author

iCal Generation

• Maps EventInstances to individual iCal events (no recurrence handling as specified)
• Event details: Uses Event.post for title/description, EventInstance for start/end times
• Location support: Includes event location from EventInstance.location.uniformly_formatted_address
• Event links: Adds "via: https://{frontend_domain}/event/{instance_id}" to descriptions (matching frontend behavior)

Technical Details

• Time conversion: Uses existing ToDbTime marshaling trait for proper timestamp handling
• Frontend domain: Leverages configured_frontend_domain() helper for correct link generation
• Error handling: Returns appropriate error messages for missing parameters or RPC failures
• Trait imports: Added EventLike trait import to enable .starts() and .ends() methods

Testing

• Unit test: Added comprehensive test for iCal generation logic
• Build verification: Code compiles successfully with only existing warnings
• Format validation: Test confirms proper iCal structure (BEGIN/END blocks, required fields)

Example Usage

# Get iCal calendar for user with ID "user123"
curl "https://yourserver.com/calendar.ics?user_id=user123"

Returns RFC 5545 compliant iCal data that can be imported into any calendar application.

Fixes #30.

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