feat(health): KPI history + sustained-backlog badge for LocalDb replication (row 33)

LocalDbOplogBacklog was reported and Prometheus-exported with nothing consuming
it: not sampled into KPI history, so a purge-burst spike could not be reviewed
after the fact, and no threshold treatment anywhere. Both halves shipped.

KPI SAMPLING. SiteHealthKpiSampleSource now emits localDbOplogBacklog and
localDbReplicationConnected (0/1), and the backlog is charted on the Health
dashboard's per-site trend panel. Charting is the point: a point-in-time tile
can never answer 'did that spike drain?', which is the only thing separating an
expected purge burst from a stalled peer.

Both are CONDITIONALLY emitted rather than coerced with ?? 0 like the
auditBacklogPending metric beside them. Both health-report fields are nullable
and null means 'no reading' — replication not configured (rig site-b/site-c) or
a failed poll — never zero. A fabricated 0 would render a pair that cannot read
its own oplog, or one deliberately unreplicated, as perfectly healthy once a
minute forever, burying the real series. So a site emits 12 metrics when
unreplicated and 14 when replicated, and a GAP in these two series truthfully
means 'not replicated / not reporting'. Note false and null differ for the
connected flag: a configured link that is DOWN samples as 0; no link samples as
nothing. Both fold as Gauge (summing an oplog depth, or a 0/1 flag, per hour is
meaningless).

THRESHOLD SEMANTICS — sustained, not instantaneous. This is the design, not a
refinement. The site_events retention purge is CDC-captured like any other
write, so the backlog jumps by the deleted batch and drains as the peer acks
it; a bare 'backlog > threshold' badge would fire on every replicated site
every day and be ignored within a week. Healthy is defined by the backlog
RETURNING to ~0, which only a dwell can express. CentralHealthAggregator holds
LocalDbBacklogAboveSince — the instant the backlog first rose above
LocalDbBacklogThreshold (default 1000) — and the badge appears only after
LocalDbBacklogSustainedFor (default 15 min). A report at or below the threshold
clears the anchor; one above leaves the ORIGINAL instant intact, so the window
is sustained rather than sliding.

The null case is deliberately a third branch: a null backlog leaves the anchor
UNCHANGED rather than clearing it. Null is evidence neither of a drain nor of a
backlog, so a node going blind mid-climb must not silently reset its own dwell,
and an unreplicated site (null forever) never accumulates an anchor and can
never be badged.

Validation rejects a non-positive threshold (0 would badge every replicated site
permanently, since an idle backlog already sits at it) and a dwell shorter than
one ReportInterval (satisfiable by a single sample, which collapses the check
back into the bare threshold it replaces).

DOC CORRECTION worth flagging: this row's premise was 'mirror the SiteAuditBacklog
configurable threshold badge'. That badge does not exist — Component-HealthMonitoring.md
described it in two places but no option, no comparison and no markup were ever
implemented. The two stale claims are corrected here rather than left to send the
next reader looking for it; the LocalDb badge is built fresh on the Metrics-stale
badge's pattern, which is a real configurable-threshold badge.

Tests: HealthMonitoring 115 (9 new aggregator tests incl. purge-burst-drains,
unreplicated-never-anchors, null-mid-climb, exact-threshold, stale-report; 5 new
validator tests; 4 new sampler tests incl. the null-semantics and false-vs-null
cases); CentralUI 1042 (5 new badge/chart tests incl. the purge-burst negative
control); Commons Kpi 93; KpiHistory 54.

docs/deployment/topology-guide.md

@@ -255,6 +255,32 @@ LocalDbReplicationConnected staying true with the backlog returning to ~0 is the
 An unreplicated node logs only the first line. If a backlog spike has no such line near it in the
 active node's log, the purge is *not* the explanation and the spike is worth investigating.
 
+**Two dashboard surfaces make this reviewable rather than a log-archaeology exercise** (register
+row 33):
+
+- **The "Replication backlog" badge** on the site tile of the Health dashboard. It is deliberately
+  a *sustained* signal, not a bare threshold: the badge appears only once the backlog has stayed
+  above `ScadaBridge:HealthMonitoring:LocalDbBacklogThreshold` (default **1000**) continuously for
+  `ScadaBridge:HealthMonitoring:LocalDbBacklogSustainedFor` (default **15 minutes**). The daily
+  purge burst described above drains in seconds to a couple of minutes, so it never reaches the
+  dwell and never badges the site — which is the entire reason the dwell exists. A bare threshold
+  would fire once a day on every replicated site and be ignored within a week. The dwell is
+  validated to be at least one `ReportInterval`, since a shorter window could be satisfied by a
+  single sample and would collapse back into a bare threshold.
+- **The "LocalDb Replication Backlog" trend chart** in the dashboard's per-site Site Health Trends
+  panel, backed by the KPI history store (sampled every minute, retained 90 days raw / 365 days
+  hourly). This is what answers the question a point-in-time reading cannot: *did that spike
+  drain?* A past purge burst appears as a spike that returns to ~0; a real fault appears as a
+  level that never comes back down.
+
+**Both surfaces are silent on an unreplicated site, by design.** `LocalDbOplogBacklog` and
+`LocalDbReplicationConnected` are nullable, and null means "no reading", never zero. The KPI
+sampler therefore writes **no sample at all** rather than a fabricated 0, so site-b/site-c show an
+empty chart rather than a flat zero line, and they never accumulate the dwell anchor the badge
+keys off. A gap in those series means "not replicated / not reporting" — which is the truth. Note
+that `false` and null differ for the connected flag: a configured link that is DOWN samples as 0,
+while no link at all samples as nothing.
+
 ### Site Pair Upgrades — stop and start BOTH nodes together
 
 **A rolling upgrade of a site pair, one node at a time, is no longer supported.** It worked while