dashboard: fix what a real fleet showed, which tests could not

Three faults, visible only once the board ran against production data.

BACKUPS SAID THE WHOLE FLEET HAD STOPPED. The lastseenat backfill was wrong. It
seeded from collectedat, reasoning that the last change was the last provable
moment - but an unchanged config writes no revision, so a machine whose settings
last changed nine months ago got a nine-month-old lastseenat and was instantly
reported as a dead backup. Every chain lit up at once, which is worse than no
card: it says the site is broken when it is fine.

The honest value is NULL. Before the column existed nothing recorded when a
config was last confirmed, and inventing a date does not change that. Migration
0003 clears the backfill, and staleness now IGNORES a NULL chain rather than
substituting timestamps that mean something else. A chain becomes measurable the
first time its PC posts, which for NTLARS is within a day.

TONER READ "None%". The supply dict has no 'percent' key - it is 'remaining'.
Supply names are also shortened, because "Black Toner Level 4%" spends three
words saying what the card already says.

THE CARDS READ AS WALLS OF TEXT. Rows wrapped into paragraphs and a card with
forty PCs pushed everything below it off the screen. Now: at most five rows with
"and N more", one line per row that truncates rather than wraps, meta pushed
right and dropped first since it matters least, and severity reduced to a small
dot beside an uppercase label instead of a coloured card - six severity-painted
cards read as a crisis, which is how a board stops being read.

Worth recording that none of this could fail in a test. Every one needed real
data on a real fleet.

plugins/backups/services/staleness.py

@@ -65,10 +65,14 @@ def stalechains(days=None, limit=50):
 
     rows = []
     for revision in latestperchain().values():
-        # Rows written before lastseenat existed fall back to the timestamps
-        # that do exist, so an old install reports something sane on day one
-        # rather than every chain at once.
-        seen = revision.lastseenat or revision.collectedat or revision.createdat
+        # NULL means never confirmed since the column existed, and it is NOT
+        # substituted with collectedat. That substitution is what the first
+        # version did, and on a real fleet it reported every machine whose
+        # config had simply been stable for months as a stopped backup - the
+        # whole board lit up and said the site was broken when it was fine.
+        # A chain becomes measurable the first time its PC posts; until then
+        # this card says nothing about it, which is the truth.
+        seen = revision.lastseenat
         if seen is None or seen >= cutoff:
             continue
         asset = db.session.get(Asset, revision.assetid)