Fix #1145: Webhook (Teams/Slack) alerts re-fire after an app restart

[erikdarlingdata]

Closes #1145.

Problem

#981 added restart-dedup for the email channel only. A restart cleared the two guards that suppress a webhook re-send, so reopening Lite re-posted a Teams/Slack alert already delivered before the restart — identical Dedup Key and Occurrences. For a webhook-only deployment there was zero restart protection.

Two independent guards must fail together: the in-memory edge-trigger watermark (#1091) resets to 0 on restart, making the gate fire; the in-memory webhook cooldown is empty, letting the post through.

Fix

1. Webhook cooldown seed — shared, BOTH apps.
WebhookAlertService now seeds its per-(serverId, metricName) cooldown from alert history on first use, mirroring the email seed (#981), via a new IAlertHistoryStore.GetLastWebhookSentUtcAsync. Lite filters notification_type IN ('webhook','email+webhook'); Dashboard filters NotificationType == "webhook". send_error is not filtered on — it tracks the email channel, so an email-failed-but-webhook-sent row must still seed. Wired into the WebhookAlertService DI in both MainWindows.

2. Edge-trigger watermark persistence — Lite.
The rolling-count gate's in-memory watermark (_lastAlertedBlockingCount / _lastAlertedDeadlockCount) reset to 0 on restart, so the first post-restart sweep re-fired for events still in the 1-hour lookback. Because that gap can exceed the cooldown (gotqn's repro was 17 min > 15-min cooldown), the time-bounded seed alone does not cover it — only persisting the watermark does. The watermark now persists to a new config_edge_trigger_watermarks DuckDB table (upsert on change), seeded before the first sweep at startup.

Why Dashboard needs no watermark persistence (parity rationale)

Dashboard's deadlock gate re-baselines on restart (raw delta) or is 5-minute-windowed (FilteredDeadlockCount, always within the cooldown the seed now covers); blocking is level + cooldown (re-fires on its normal cadence, now bounded by the seeded cooldown). None produces the byte-identical duplicate the Lite edge-trigger gate does. The webhook cooldown seed (part 1) is the shared half and applies to both apps.

Tests

• Lite 505 + Dashboard 487 green; 0 warnings.
• New: webhook-row history filter (GetLastWebhookSentUtc_FiltersToWebhookRows_IncludingEmailWebhook); watermark save/load/upsert round-trip (EdgeTriggerWatermark_SaveLoad_RoundTripsAndUpserts); WebhookAlertService seed-suppresses-within-window / seed-older-than-cooldown-does-not-suppress / null-store-attempts.

🤖 Generated with Claude Code