feat(#1008): archive auto-purge + dedicated archives:purge permission

Adds an archive counterpart to the library trash sweeper shipped in the
  previous commit. Unlike the library flow, archives are hard-deleted —
  print history is a decaying timeline, so there is no trash intermediate;
  download or favourite anything you want to keep first.

  Backend
  - New ArchivePurgeService (backend/app/services/archive_purge.py) with
    its own 15-minute scheduler loop and a 24h throttle on actual purge
    runs. Delegates every delete to the existing safety-checked
    ArchiveService.delete_archive so the 3MF, thumbnail, timelapse, source
    3MF, F3D, and photo folder all get cleaned up together with the DB
    row. Per-row session via async_session() avoids commit-per-row churn
    on any caller-passed session.
  - New /archives/purge/{preview,settings} + POST /archives/purge routes
    gated on a dedicated archives:purge permission (not archives:delete_all)
    so admins can delegate bulk-delete to a role without granting
    per-archive delete on other users' rows.
  - seed_default_groups() now backfills both library:purge and
    archives:purge on the Administrators group for upgraded installs —
    the original library:purge was added after Administrators was first
    seeded so the "create if not exists" path skipped existing DBs and
    left admins without the permission.
  - 8 new integration tests (defaults, settings roundtrip, bound
    validation, preview, manual purge, auto-purge enabled path, 24h
    throttle, disabled skip).

  Frontend
  - Settings → Archives card gains an auto-purge toggle + age input (7d
    floor, 10y ceiling, 365d default), with a save-toast on every change.
    The bulk "Purge old" button lives on the Archives page header
    (rightmost, after Upload 3MF) to match the File Manager pattern —
    configuration in Settings, one-shot action on the page.
  - New PurgeArchivesModal mirrors PurgeOldFilesModal: live preview (count
    + total size freed + sample filenames) debounced at 300ms, amber
    "hard-delete, no undo" warning.
  - Admin-only UI gates on archives:purge via the standard hasPermission
    hook; Permission TS union updated.
  - i18n blocks across all 8 locales (en/de full, other 6 English
    fallback per project convention).

  Docs
  - CHANGELOG entry under 0.2.4b1 following the existing library-trash
    entry.
  - bambuddy-wiki archiving.md gains a new "Auto-Purge" section.
  - bambuddy-website features.html gets a matching bullet.

  Verification: python -m ruff check backend/app/ clean; 25 integration
  tests pass (8 archive_purge + 17 library_trash regression); npm run
  build clean.

backend/app/api/routes/archive_purge.py

@@ -0,0 +1,81 @@
+"""Archive auto-purge endpoints (#1008 follow-up).
+
+Admin-only (``ARCHIVES_PURGE``). Provides:
+
+* ``GET /archives/purge/preview`` — live count for the admin slider
+* ``POST /archives/purge`` — one-shot manual bulk delete
+* ``GET/PUT /archives/purge/settings`` — auto-purge toggle + threshold
+"""
+
+from __future__ import annotations
+
+import logging
+
+from fastapi import APIRouter, Depends, HTTPException, Query
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from backend.app.core.auth import require_permission_if_auth_enabled
+from backend.app.core.database import get_db
+from backend.app.core.permissions import Permission
+from backend.app.models.user import User
+from backend.app.schemas.archive_purge import (
+    ArchivePurgePreviewResponse,
+    ArchivePurgeRequest,
+    ArchivePurgeResponse,
+    ArchivePurgeSettings,
+)
+from backend.app.services.archive_purge import (
+    MAX_AUTO_PURGE_DAYS,
+    MIN_AUTO_PURGE_DAYS,
+    archive_purge_service,
+)
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/archives", tags=["archives-purge"])
+
+
+@router.get("/purge/preview", response_model=ArchivePurgePreviewResponse)
+async def preview_archive_purge(
+    older_than_days: int = Query(ge=1, le=3650),
+    db: AsyncSession = Depends(get_db),
+    _: User | None = Depends(require_permission_if_auth_enabled(Permission.ARCHIVES_PURGE)),
+):
+    """Count + size of archives eligible for purge. Read-only."""
+    result = await archive_purge_service.preview_purge(db, older_than_days=older_than_days)
+    return ArchivePurgePreviewResponse(**result)
+
+
+@router.post("/purge", response_model=ArchivePurgeResponse)
+async def execute_archive_purge(
+    body: ArchivePurgeRequest,
+    db: AsyncSession = Depends(get_db),
+    _: User | None = Depends(require_permission_if_auth_enabled(Permission.ARCHIVES_PURGE)),
+):
+    """Hard-delete archives older than the threshold. Irreversible."""
+    deleted = await archive_purge_service.purge_older_than(db, older_than_days=body.older_than_days)
+    return ArchivePurgeResponse(deleted=deleted)
+
+
+@router.get("/purge/settings", response_model=ArchivePurgeSettings)
+async def get_archive_purge_settings(
+    db: AsyncSession = Depends(get_db),
+    _: User | None = Depends(require_permission_if_auth_enabled(Permission.ARCHIVES_PURGE)),
+):
+    cfg = await archive_purge_service.get_settings(db)
+    return ArchivePurgeSettings(enabled=cfg["enabled"], days=cfg["days"])
+
+
+@router.put("/purge/settings", response_model=ArchivePurgeSettings)
+async def update_archive_purge_settings(
+    body: ArchivePurgeSettings,
+    db: AsyncSession = Depends(get_db),
+    _: User | None = Depends(require_permission_if_auth_enabled(Permission.ARCHIVES_PURGE)),
+):
+    if body.days < MIN_AUTO_PURGE_DAYS or body.days > MAX_AUTO_PURGE_DAYS:
+        raise HTTPException(
+            status_code=400,
+            detail=f"days must be between {MIN_AUTO_PURGE_DAYS} and {MAX_AUTO_PURGE_DAYS}",
+        )
+    saved = await archive_purge_service.set_settings(db, enabled=body.enabled, days=body.days)
+    return ArchivePurgeSettings(enabled=saved["enabled"], days=saved["days"])