Add user merge endpoint and UI (Zusammenführen von Kunden)

[Copilot]

Admins need the ability to consolidate duplicate user accounts, transferring all associated data to a target user and soft-deleting the source.

Backend — POST /api/v1/admin/users/{user_id}/merge

{ "source_user_id": 42 }

Transfers from source → target user:

• Sessions, API keys, OAuth identities
• Spool events, filament ratings (conflict resolution: keep target's rating when both rated the same filament)
• Roles (union, deduplicated)

Guards:

• admin:users_manage permission required
• Rejects self-merge (400)
• Rejects merging the last superadmin as source (400)
• Returns UserDetailResponse of the updated target

Frontend — Admin Users page

• Merge button added to each row (hidden for self)
• Modal shows source user (read-only) + dropdown of eligible target users
• Confirmation dialog before execution

i18n

New keys added to both en.json and de.json under admin.* (mergeUser, mergeUserTitle, mergeUserDesc, mergeConfirm, etc.).

Tests

tests/test_merge_users.py covers: successful merge + data transfer, self-merge rejection, missing source/target (404), permission enforcement, last-superadmin guard, and role deduplication.

Original prompt

zusammen führen von kunden

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.