feat: provider-agnostic telephony integration with Plivo support

[aadhar-EAI]

Summary

• Adds a full provider-agnostic telephony layer with Plivo as the first supported carrier, designed to be extensible for future providers (Twilio, Telnyx, Exotel, etc.)
• Introduces org-level telephony configuration, phone number inventory management, outbound calling, voice OTP verification, and number masking capabilities
• Links telephony phone numbers to agents so they can make and receive calls over PSTN

What's Included

Backend:

• app/services/telephony/ — Plivo client wrapper, XML helper, and provider-agnostic TelephonyService with outbound calls, OTP verify, number masking, and webhook handling
• app/api/v1/routes/telephony.py — Full REST API: config CRUD, number sync/list/patch, outbound calls, verify sessions, masking, and Plivo webhooks (answer, events, masking)
• app/models/database.py / enums.py / schemas.py — TelephonyIntegration, TelephonyPhoneNumber, TelephonyVerifySession, TelephonyMaskedSession models with TelephonyProvider enum
• app/migrations/016-018 — Three sequential migrations: initial Plivo tables, rename to provider-agnostic schema, add telephony link to agents
• app/config.py — Plivo global config fields (auth_id, auth_token, verify_app_uuid, webhook_base_url)
• Agent model extended with telephony_phone_number_id foreign key

Frontend:

• Telephony provider configuration UI in Integrations page with encrypted credential storage
• Phone number sync, listing, and agent assignment in agent create/edit forms
• Provider config display with connection testing

Test plan

• Create telephony config with Plivo credentials and verify test_connection succeeds
• Sync phone numbers from Plivo and verify they appear in the number list
• Assign a phone number to an agent via the edit form
• Initiate an outbound call via POST /telephony/calls/outbound and verify call is placed
• Verify answer and event webhooks handle inbound call routing correctly
• Confirm migrations run cleanly on a fresh database

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