Surface recovered Invalid channels on the dashboard, wire --include-invalid into the deep check

Follow-up to the previous commit on this branch: that one added
--invalid/--include-invalid to check_channels.py but deliberately left
them unwired, because generate_dashboard.py's "most recent row per
(list, channel)" view assumed every row was a working channel -
turning --include-invalid on in a workflow would have bloated the
"needs attention" table with hundreds of already-known-dead Invalid
rows, while hiding the one thing that's actually interesting: an
Invalid channel that's alive again wouldn't show up there at all,
since that table only lists rows that are *not* alive.

- generate_dashboard.py: every existing count (cards, trend, by-list,
  "needs attention") is now built from `>` (working) rows only, via a
  `marker_of()` helper that treats history written before this field
  existed as `>` too, since only working channels were ever checked
  back then - so this is a no-op for every dashboard that hasn't
  started using --include-invalid yet.
- New "Flagged Invalid, answering again" section lists `x` rows whose
  latest check came back alive/flaky - the dashboard's equivalent of
  check_channels.py's own `RECOVERED?` line - with a note that this is
  a shortlist for a human, not something to auto-restore.
- check_channels_deep.yml now runs with --include-invalid, and its
  step-summary blurb mentions the new section.

Tested: wrote a small synthetic history.jsonl mixing a marker-less
legacy row, `>` rows, and `x` rows (one alive, one dead) and confirmed
the channel count/cards only reflect the `>` rows, the alive `x` row
shows up under "Flagged Invalid, answering again", the dead `x` row
shows up nowhere (expected - it's just confirming the status quo), and
an all-`>` history renders the new section's "none right now" empty
state. Also ran it against this repo's real 77MB
.github/checker-history/history.jsonl (all pre-dates this change, so
no `x` rows in it) - renders in ~1.5s, unchanged output. pylint:
10.00/10 on both files.

.github/workflows/check_channels_deep.yml

@@ -28,7 +28,7 @@ jobs:
 
       - name: Run the deep check (HTTP probes + ffprobe confirmation on dead channels)
         run: |
-          python3 check_channels.py --attempts 3 --timeout 12 --pause 5 --workers 40 --confirm-dead --json run.jsonl | tee check_summary.txt
+          python3 check_channels.py --attempts 3 --timeout 12 --pause 5 --workers 40 --confirm-dead --include-invalid --json run.jsonl | tee check_summary.txt
 
       - name: Append to history and prune entries older than 90 days
         run: |
@@ -73,7 +73,7 @@ jobs:
           {
             echo "## Deep channel check - $(date -u +%Y-%m-%dT%H:%M:%SZ)"
             echo ""
-            echo "Channels that looked \`dead\` over HTTP were given a second opinion by \`ffprobe\`. A channel still reported \`dead\` here survived both checks and is safe to act on; one reported \`disputed\` looked dead over HTTP but ffprobe found a real stream behind it - needs a human look, not an automatic edit."
+            echo "Channels that looked \`dead\` over HTTP were given a second opinion by \`ffprobe\`. A channel still reported \`dead\` here survived both checks and is safe to act on; one reported \`disputed\` looked dead over HTTP but ffprobe found a real stream behind it - needs a human look, not an automatic edit. This run also re-checks every channel already flagged Invalid (\`--include-invalid\`) - one reported \`RECOVERED?\` answered like a live stream again and shows up on the dashboard's own 'Flagged Invalid, answering again' section, also pending a human look before anyone restores it."
             echo ""
             echo '```'
             cat check_summary.txt