F-14 [Medium] Terraform stores raw SQL admin password in Key Vault connection string — use Managed Identity instead

## Summary\n\n`infra/terraform/main.tf` stores `azurerm_key_vault_secret.sql_connection_string` with a plain `Password=...` embedded in the connection string. The application already supports Managed Identity authentication for SQL, making the stored password unnecessary.\n\n## Affected Files\n\n- `infra/terraform/main.tf` — `azurerm_key_vault_secret.sql_connection_string`\n\n## Impact\n\nSQL admin password is stored in Key Vault in plain text. Unnecessary credential sprawl and elevated risk if the secret is accessed by unauthorized principals.\n\n## Fix\n\n- Switch to a Managed Identity connection string (no password required).\n- Remove the password from the Key Vault secret.\n\n## References\n\n- Audit artifact: `spin-agent-full-audit.md` (project artifact 1e910fcc580a4cb0)\n- Auditor: Thor

Open infra/terraform/main.tf and inspect the azurerm_key_vault_secret.sql_connection_string definition first. Confirm how the application’s existing Managed Identity SQL authentication is represented, then validate the Terraform configuration and check that the resulting secret contains no password while still providing the required connection details.

Written by the indexing model from the issue text.