Add jail distribution chart (Stage 5)

- backend: GET /api/dashboard/bans/by-jail endpoint
  - JailBanCount + BansByJailResponse Pydantic models in ban.py
  - bans_by_jail() service function with origin filter support
  - Route added to dashboard router
  - 17 new tests (7 service, 10 router); full suite 497 passed, 83% coverage

- frontend: JailDistributionChart component
  - JailBanCount / BansByJailResponse types in types/ban.ts
  - dashboardBansByJail endpoint constant in api/endpoints.ts
  - fetchBansByJail() in api/dashboard.ts
  - useJailDistribution hook in hooks/useJailDistribution.ts
  - JailDistributionChart component (horizontal bar chart, Recharts)
  - DashboardPage: full-width Jail Distribution section below Top Countries

backend/app/models/ban.py

@@ -280,3 +280,29 @@ class BanTrendResponse(BaseModel):
         ...,
         description="Human-readable bucket size label (e.g. '1h', '6h', '1d', '7d').",
     )
+
+
+# ---------------------------------------------------------------------------
+# By-jail endpoint models
+# ---------------------------------------------------------------------------
+
+
+class JailBanCount(BaseModel):
+    """A single jail entry in the bans-by-jail aggregation."""
+
+    model_config = ConfigDict(strict=True)
+
+    jail: str = Field(..., description="Jail name.")
+    count: int = Field(..., ge=0, description="Number of bans recorded in this jail.")
+
+
+class BansByJailResponse(BaseModel):
+    """Response for the ``GET /api/dashboard/bans/by-jail`` endpoint."""
+
+    model_config = ConfigDict(strict=True)
+
+    jails: list[JailBanCount] = Field(
+        default_factory=list,
+        description="Jails ordered by ban count descending.",
+    )
+    total: int = Field(..., ge=0, description="Total ban count in the selected window.")