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/models/ban.py

@@ -3,6 +3,7 @@
 Request, response, and domain models used by the ban router and service.
 """
 
+import math
 from typing import Literal
 
 from pydantic import BaseModel, ConfigDict, Field
@@ -220,3 +221,62 @@ class BansByCountryResponse(BaseModel):
         description="All bans in the selected time window (up to the server limit).",
     )
     total: int = Field(..., ge=0, description="Total ban count in the window.")
+
+
+# ---------------------------------------------------------------------------
+# Trend endpoint models
+# ---------------------------------------------------------------------------
+
+#: Bucket size in seconds for each time-range preset.
+BUCKET_SECONDS: dict[str, int] = {
+    "24h": 3_600,          # 1 hour  → 24 buckets
+    "7d": 6 * 3_600,       # 6 hours → 28 buckets
+    "30d": 86_400,         # 1 day   → 30 buckets
+    "365d": 7 * 86_400,    # 7 days  → ~53 buckets
+}
+
+#: Human-readable bucket size label for each time-range preset.
+BUCKET_SIZE_LABEL: dict[str, str] = {
+    "24h": "1h",
+    "7d": "6h",
+    "30d": "1d",
+    "365d": "7d",
+}
+
+
+def bucket_count(range_: TimeRange) -> int:
+    """Return the number of buckets needed to cover *range_* completely.
+
+    Args:
+        range_: One of the supported time-range presets.
+
+    Returns:
+        Ceiling division of the range duration by the bucket size so that
+        the last bucket is included even when the window is not an exact
+        multiple of the bucket size.
+    """
+    return math.ceil(TIME_RANGE_SECONDS[range_] / BUCKET_SECONDS[range_])
+
+
+class BanTrendBucket(BaseModel):
+    """A single time bucket in the ban trend series."""
+
+    model_config = ConfigDict(strict=True)
+
+    timestamp: str = Field(..., description="ISO 8601 UTC start of the bucket.")
+    count: int = Field(..., ge=0, description="Number of bans that started in this bucket.")
+
+
+class BanTrendResponse(BaseModel):
+    """Response for the ``GET /api/dashboard/bans/trend`` endpoint."""
+
+    model_config = ConfigDict(strict=True)
+
+    buckets: list[BanTrendBucket] = Field(
+        default_factory=list,
+        description="Time-ordered list of ban-count buckets covering the full window.",
+    )
+    bucket_size: str = Field(
+        ...,
+        description="Human-readable bucket size label (e.g. '1h', '6h', '1d', '7d').",
+    )