fix(unifi): the Overview's history buckets on Postgres; devices name their state (#5207)

#5207 "UniFi Overview tab: func.strftime is SQLite-only": the WAN and
client sparklines bucketed with strftime('%s'), which Postgres lacks, so
/plugins/unifi/rows failed. Traefik's date_bin bucket moves to
core/time_range.time_bucket and both plugins use it. Devices list offline
and failing first, and a state other than online is named (Upgrading,
Adopting, Heartbeat missed...) instead of drawn as Down.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/core/time_range.py

@@ -4,6 +4,8 @@ from __future__ import annotations
 from datetime import datetime, timedelta, timezone
 from typing import TypeVar
 
+from sqlalchemy import func
+
 RANGES: dict[str, timedelta] = {
     "1h":  timedelta(hours=1),
     "6h":  timedelta(hours=6),
@@ -33,6 +35,16 @@ def bucket_seconds(since: datetime, target_points: int = 80) -> int:
     return max(60, int(range_secs / target_points))
 
 
+def time_bucket(column, seconds: int):
+    """The start of the `seconds`-wide window a timestamp falls in, aligned to
+    the epoch, as a Postgres timestamp labelled "bucket". Group and order by
+    it. (An integer epoch divided by an int rendered as numeric division in
+    Postgres, so no two scrapes shared a bucket, and overflowed int4 after
+    2038 (#5256); strftime('%s') is SQLite-only (#5207).)"""
+    return func.date_bin(func.make_interval(0, 0, 0, 0, 0, 0, int(seconds)), column,
+                         func.to_timestamp(0)).label("bucket")
+
+
 T = TypeVar("T")