fix(system): three next-runs, completed-date data_date, running mark

Split /system schedule into fans / snapshot+UP / discovery next-runs.
Drive data_date and snapshot_days from fully completed video snapshots
only, keep prior success date on failure, show task failure records, and
mark in-progress tasks as 运行中 without wall-clock next-run jumps.

tools/seed_preview_db.py

@@ -278,6 +278,13 @@ def _seed_task_records(
             "error": None,
             "summary": {"refs": 2, "added": 2, "ok": True},
         },
+        "fans": {
+            "name": "fans", "ok": True,
+            "started_at": int(base + 1124),
+            "finished_at": int(base + 1130),
+            "error": None,
+            "summary": {"fans_ok": 1, "fans_failed": 0, "fans_skipped": 0},
+        },
         "discovery": {
             "name": "discovery", "ok": True,
             "started_at": int(base + 21 * 3600),
@@ -309,6 +316,17 @@ def _seed_task_records(
         "INSERT OR REPLACE INTO kv(k, v) VALUES ('snapshot:last_completed', ?)",
         (latest_day.isoformat(),),
     )
+    conn.execute(
+        "INSERT OR REPLACE INTO kv(k, v) VALUES (?, '1')",
+        (f"snapshot:completed:{latest_day.isoformat()}",),
+    )
+    for (day,) in conn.execute(
+        "SELECT DISTINCT stat_date FROM snapshot WHERE ok = 1"
+    ).fetchall():
+        conn.execute(
+            "INSERT OR REPLACE INTO kv(k, v) VALUES (?, '1')",
+            (f"snapshot:completed:{day}",),
+        )
     conn.execute(
         "INSERT OR REPLACE INTO kv(k, v) VALUES (?, '1')",
         (f"snapshot:backup:{latest_day.isoformat()}",),