fix: return 409 Conflict instead of 500 when creating a duplicate admin or realm

[pointche-ens]

Summary

• POST /admins and POST /admins/realms used to leak a raw 500 with database internals when the target ID already existed (e.g. re-creating the seeded admin user, or an existing realm), instead of a clean 409 Conflict.
• This mirrors the fix already applied to POST /realms/{realm_id}/userpass (AuthDbError::from_insert_error), which hadn't been extended to the admin/realm creation paths.
• Applied to all three database backends (PostgreSQL, MySQL, SQLite).

[Copilot]

🟢 Approval recommended

The core behavior change is correctly implemented across all backends and covered by targeted tests, with only a minor test naming/doc wording nit noted.

Pull request overview

This PR updates the database insert paths for realm and admin creation so that attempting to create an already-existing realm/admin returns a clean 409 Conflict (instead of bubbling up a 500 with database error internals), aligning behavior across SQLite/PostgreSQL/MySQL and strengthening the API’s error contract.

Changes:

• Map unique-constraint insert failures to AuthDbError::Conflict for create_realm and create_admin across SQLite/PostgreSQL/MySQL.
• Add/adjust integration tests to assert duplicate realm/admin creation yields 409.
• Add a branch changelog entry documenting the behavior change.

File summaries

File	Description
server/src/tests/super_admin_api.rs	Adds a test ensuring duplicate realm creation returns 409 Conflict.
server/src/tests/admin_api.rs	Tightens the duplicate admin creation test to explicitly assert 409 (and updates imports).
server/src/database/impls/sqlite.rs	Maps duplicate realm/admin insert errors to Conflict via AuthDbError::from_insert_error.
server/src/database/impls/postgres.rs	Same as SQLite for PostgreSQL backend.
server/src/database/impls/mysql.rs	Same as SQLite for MySQL backend.
CHANGELOG/fix_admin-realm-create-conflict.md	Documents the 500→409 behavior change for duplicate admin/realm creation.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 1
• Review effort level: Lite

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