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.

tests/web_fixtures.py

@@ -166,6 +166,8 @@ def build_db(tmp: str | Path) -> Path:
         repo.mark_candidate_decision(202, "rejected", now=ts("2026-10-30"))
         repo.finish_folder_sync(FOLDER, media_count=3, full_refresh=True)
         repo.kv_set("snapshot:last_completed", DATES[-1])
+        for day in DATES:
+            repo.kv_set(f"snapshot:completed:{day}", "1")
         repo.kv_set("auth:ok", "1")
         repo.kv_set("auth:message", "OK")
         repo.kv_set(
@@ -176,6 +178,22 @@ def build_db(tmp: str | Path) -> Path:
                 "error": None, "summary": {"completed": True},
             }),
         )
+        repo.kv_set(
+            "task:last:fans",
+            json.dumps({
+                "name": "fans", "ok": True,
+                "started_at": ts(DATES[-1]) + 60, "finished_at": ts(DATES[-1]) + 90,
+                "error": None, "summary": {"fans_ok": 1, "fans_failed": 0},
+            }),
+        )
+        repo.kv_set(
+            "task:last:discovery",
+            json.dumps({
+                "name": "discovery", "ok": True,
+                "started_at": ts(DATES[-1]) - 3600, "finished_at": ts(DATES[-1]) - 3500,
+                "error": None, "summary": {"candidates": 0},
+            }),
+        )
     finally:
         db.close()
     return path