feat: support web calls in webhook

[cubic-dev-local]

## What does this PR do?

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• N/A I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

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Based on: calcom/cal.com#23788

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Summary by cubic

Adds support for Retell web calls in the webhook. Web calls now charge credits to the owning agent’s user/team even when no phone numbers are present.

• New Features
  • Accepts web_call events; phone fields and direction are optional in the payload.
  • Resolves agent_id via PrismaAgentRepository to map credits to the correct user/team; phone calls still use from_number lookup.
  • Centralizes credit charging with a configurable per-minute rate (default $0.29) and rounds up; improved logging and error handling. Skips inbound calls only for phone calls.
  • Adds tests for web calls: user and team agents, no phone number, missing agent_id, agent not found, and credit charging.

[cubic-dev-ai]

No issues found across 2 files

[github-actions]

This PR is being marked as stale due to inactivity.