docs: add OpenAPI responses={} to all router endpoints

Add explicit HTTP status code documentation to every endpoint
across 15 router files. Each endpoint now declares all possible
response codes (200/201/204/400/401/404/409/429/502/503) with
descriptions so frontend can distinguish error types.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/routers/dashboard.py

@@ -56,6 +56,11 @@ _DEFAULT_RANGE: TimeRange = "24h"
     "/status",
     response_model=ServerStatusResponse,
     summary="Return the cached fail2ban server status",
+    responses={
+        200: {"description": "Server status returned", "model": ServerStatusResponse},
+        401: {"description": "Session missing, expired, or invalid"},
+        502: {"description": "fail2ban unreachable"},
+    },
 )
 async def get_server_status(
     server_status: ServerStatusDep,
@@ -84,6 +89,11 @@ async def get_server_status(
     "/bans",
     response_model=DashboardBanListResponse,
     summary="Return a paginated list of recent bans",
+    responses={
+        200: {"description": "Ban list returned", "model": DashboardBanListResponse},
+        401: {"description": "Session missing, expired, or invalid"},
+        502: {"description": "fail2ban unreachable"},
+    },
 )
 async def get_dashboard_bans(
     _auth: AuthDep,
@@ -145,6 +155,11 @@ async def get_dashboard_bans(
     "/bans/by-country",
     response_model=BansByCountryResponse,
     summary="Return ban counts aggregated by country",
+    responses={
+        200: {"description": "Ban counts by country returned", "model": BansByCountryResponse},
+        401: {"description": "Session missing, expired, or invalid"},
+        502: {"description": "fail2ban unreachable"},
+    },
 )
 async def get_bans_by_country(
     _auth: AuthDep,
@@ -205,6 +220,11 @@ async def get_bans_by_country(
     "/bans/trend",
     response_model=BanTrendResponse,
     summary="Return ban counts aggregated into time buckets",
+    responses={
+        200: {"description": "Ban trend data returned", "model": BanTrendResponse},
+        401: {"description": "Session missing, expired, or invalid"},
+        502: {"description": "fail2ban unreachable"},
+    },
 )
 async def get_ban_trend(
     _auth: AuthDep,
@@ -259,6 +279,11 @@ async def get_ban_trend(
     "/bans/by-jail",
     response_model=BansByJailResponse,
     summary="Return ban counts aggregated by jail",
+    responses={
+        200: {"description": "Ban counts by jail returned", "model": BansByJailResponse},
+        401: {"description": "Session missing, expired, or invalid"},
+        502: {"description": "fail2ban unreachable"},
+    },
 )
 async def get_bans_by_jail(
     _auth: AuthDep,