feat: show RDP session duration in Overview and Events (0.5.16)

Add session_duration_sec to EventSummary and a Duration column (HH:MM:SS / Nd HH:MM:SS).

backend/app/services/session_duration.py

@@ -0,0 +1,37 @@
+"""Format and extract RDP/RDG session_duration_sec for UI."""
+
+from __future__ import annotations
+
+from typing import Any
+
+
+def extract_session_duration_sec(details: dict[str, Any] | None) -> int | None:
+    if not isinstance(details, dict):
+        return None
+    raw = details.get("session_duration_sec")
+    if raw is None or raw == "":
+        return None
+    try:
+        value = int(raw)
+    except (TypeError, ValueError):
+        return None
+    if value < 0:
+        return None
+    return value
+
+
+def format_session_duration(seconds: int) -> str:
+    """
+    Human-readable session length for lists:
+    - under 24h → HH:MM:SS (05:05:24)
+    - 24h+ → Nд HH:MM:SS (1д 00:01:25)
+    """
+    if seconds < 0:
+        return "—"
+    days, rem = divmod(int(seconds), 86_400)
+    hours, rem = divmod(rem, 3600)
+    minutes, secs = divmod(rem, 60)
+    clock = f"{hours:02d}:{minutes:02d}:{secs:02d}"
+    if days:
+        return f"{days}д {clock}"
+    return clock