feat(monitor): appendfsync=always main-thread blocking advisory (valkey#3515)

[jamby77]

Summary

Adds the AOF fsync-policy advisory from valkey-io/valkey#3515 to the config-hazard subsystem (the sibling of #337's valkey#3983 hazard): with appendfsync always, every write fsyncs synchronously on the main thread, so disk latency becomes command latency. The broader upstream AOF-modernization work (WAL headers, io_uring, direct I/O) is unshipped and not pollable — the misconfiguration and its symptom are observable today.

Detection (per the issue's guardrails)

• appendonly=no → no-op (managed/ephemeral instances with AOF intentionally off never fire).
• appendfsync=always with no symptoms → low-severity advisory (severity: info, status: advisory): the config is a latency risk, consider everysec unless per-write durability is a hard requirement.
• Escalates to a warning hazard when symptoms confirm blocking, and the message names the specific counters: aof_delayed_fsync rising across probes, aof-fsync-always/aof-write LATENCY events, or aof_last_write_status != ok.
• appendfsync=everysec stays quiet unless aof_delayed_fsync climbs on two consecutive probes (the once-per-second background fsync itself backing up).
• Rate limiting comes from the subsystem's design: findings are TTL-cached polled state (60s) surfaced via health → dashboard banner, not repeated events.

Changes

• evaluateAppendfsyncHazard pure evaluator in config-hazard.ts; ConfigHazardFinding widened (new ids, info severity, advisory status).
• ConfigHazardService probes appendfsync, INFO persistence, and LATENCY LATEST on the existing TTL-cached path, tracking a per-connection rising streak for aof_delayed_fsync. Symptom probe failures degrade to config-only evaluation instead of suppressing the advisory.
• ConfigHazardBanner gains an advisory presentation (info icon, muted border) alongside hazard/unverified.

Test plan

• 9 pure evaluator tests (acceptance matrix: AOF off, always clean/rising/latency-event/write-status, everysec healthy/single-rise/steady-climb, unknown policies).
• 7 service tests (probe wiring, cross-probe streak escalation, everysec two-probe gate, LATENCY failure tolerance, coexistence with the valkey#3983 finding); cache tests updated to count probes rather than raw CONFIG GET calls.
• 2 new banner tests (advisory rendering, escalated hazard rendering).
• monitor suite 346/346, health + MCP health 7/7, banner 6/6; tsc --noEmit clean for api and web.

Closes #368

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Note

Medium Risk
Touches health-polling probe paths and shared health types; logic is well-tested but adds more Redis commands per poll when AOF is enabled and changes what the dashboard surfaces.

Overview
Extends the config-hazard monitor with valkey#3515: when AOF is on and appendfsync=always, every write fsyncs on the main thread, so disk latency can stall commands.

Detection behavior: appendonly=no stays silent. appendfsync=always with no symptoms yields a low-severity advisory (info / advisory); escalation to a warning hazard uses rising aof_delayed_fsync across probes, fresh aof-fsync-always / aof-write LATENCY events (5‑minute freshness), or aof_last_write_status not ok. everysec only fires after two consecutive rising aof_delayed_fsync probes (background fsync backlog), not on config alone.

Implementation: Pure evaluateAppendfsyncHazard in config-hazard.ts; ConfigHazardService adds CONFIG appendfsync, INFO persistence, and LATENCY LATEST on the existing 60s TTL cache, with per-connection streak tracking. Failed INFO/LATENCY probes degrade to config-only advisory rather than hiding findings. Shared ConfigHazardFinding types add new ids, info severity, and advisory status. ConfigHazardBanner shows advisories with muted styling and an info icon alongside hazard/unverified.

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[jamby77]

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