Add GET /api/dashboard/bans/trend endpoint

Implement time-bucketed ban aggregation for dashboard trend charts:

- Add BanTrendBucket / BanTrendResponse Pydantic models and
  BUCKET_SECONDS / BUCKET_SIZE_LABEL / bucket_count helpers to ban.py
- Add ban_service.ban_trend(): queries fail2ban DB with SQL bucket
  grouping, fills zero-count buckets, respects origin filter
- Add GET /api/dashboard/bans/trend route in dashboard.py
- 20 new tests (10 service, 10 router); 480 total pass, 83% coverage
- ruff + mypy --strict clean

backend/app/routers/dashboard.py

@@ -4,7 +4,9 @@ Provides the ``GET /api/dashboard/status`` endpoint that returns the cached
 fail2ban server health snapshot.  The snapshot is maintained by the
 background health-check task and refreshed every 30 seconds.
 
-Also provides ``GET /api/dashboard/bans`` for the dashboard ban-list table.
+Also provides ``GET /api/dashboard/bans`` for the dashboard ban-list table,
+``GET /api/dashboard/bans/by-country`` for country aggregation, and
+``GET /api/dashboard/bans/trend`` for time-bucketed ban counts.
 """
 
 from __future__ import annotations
@@ -20,6 +22,7 @@ from app.dependencies import AuthDep
 from app.models.ban import (
     BanOrigin,
     BansByCountryResponse,
+    BanTrendResponse,
     DashboardBanListResponse,
     TimeRange,
 )
@@ -161,3 +164,45 @@ async def get_bans_by_country(
         origin=origin,
     )
 
+
+@router.get(
+    "/bans/trend",
+    response_model=BanTrendResponse,
+    summary="Return ban counts aggregated into time buckets",
+)
+async def get_ban_trend(
+    request: Request,
+    _auth: AuthDep,
+    range: TimeRange = Query(default=_DEFAULT_RANGE, description="Time-range preset."),
+    origin: BanOrigin | None = Query(
+        default=None,
+        description="Filter by ban origin: 'blocklist' or 'selfblock'. Omit for all.",
+    ),
+) -> BanTrendResponse:
+    """Return ban counts grouped into equal-width time buckets.
+
+    Each bucket represents a contiguous time interval within the selected
+    window.  All buckets are returned — empty buckets (zero bans) are
+    included so the frontend always receives a complete, gap-free series
+    suitable for rendering a continuous area or line chart.
+
+    Bucket sizes:
+
+    * ``24h``  → 1-hour buckets  (24 total)
+    * ``7d``   → 6-hour buckets  (28 total)
+    * ``30d``  → 1-day buckets   (30 total)
+    * ``365d`` → 7-day buckets   (~53 total)
+
+    Args:
+        request: The incoming request (used to access ``app.state``).
+        _auth: Validated session dependency.
+        range: Time-range preset.
+        origin: Optional filter by ban origin.
+
+    Returns:
+        :class:`~app.models.ban.BanTrendResponse` with the ordered bucket
+        list and the bucket-size label.
+    """
+    socket_path: str = request.app.state.settings.fail2ban_socket
+
+    return await ban_service.ban_trend(socket_path, range, origin=origin)