fix(overview): one bad health response no longer stops the dashboard host (scadaproj#29)

ZbHealthReportClient caught only HttpRequestException/OperationCanceledException,
so an unknown Content-Type charset (InvalidOperationException from
ReadAsStringAsync) escaped the probe, faulted Task.WhenAll and the poller
BackgroundService, and StopHost took the dashboard down.

- client: final catch-all returns an Unreachable result for that instance
- poller: per-target Guarded probe + catch-and-log per sweep in ExecuteAsync
- tests: unknown charset (both tiers), arbitrary handler exception, sweep with
  one bad instance, and the BackgroundService survival loop
- code-reviews: Overview-001 Resolved; README regenerated

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

code-reviews/README.md

@@ -40,7 +40,7 @@ code-reviews/
 
 ## Summary
 
-10 of 10 libraries reviewed. 49 pending findings across all libraries.
+10 of 10 libraries reviewed. 48 pending findings across all libraries.
 
 Coverage: Secrets, LocalDb, GalaxyRepository and Overview first reviewed, and Auth, Health,
 Theme, Audit, Configuration and Telemetry re-reviewed, on 2026-11-09 (#15). Each finding also
@@ -49,10 +49,10 @@ carries a P1/P2/P3 priority (P1 = Critical/High, P2 = Medium, P3 = Low).
 | Severity | Open findings |
 |----------|---------------|
 | Critical | 0 |
-| High | 1 |
+| High | 0 |
 | Medium | 15 |
 | Low | 33 |
-| **Total** | **49** |
+| **Total** | **48** |
 
 ## Library Status
 
@@ -64,7 +64,7 @@ carries a P1/P2/P3 priority (P1 = Critical/High, P2 = Medium, P3 = Low).
 | [GalaxyRepository](GalaxyRepository/findings.md) | 2026-11-09 | `91fb2b4` | 0/0/2/6 | 8 | 8 |
 | [Health](Health/findings.md) | 2026-11-09 | `91fb2b4` | 0/0/0/1 | 1 | 7 |
 | [LocalDb](LocalDb/findings.md) | 2026-11-09 | `91fb2b4` | 0/0/4/3 | 7 | 7 |
-| [Overview](Overview/findings.md) | 2026-11-09 | `91fb2b4` | 0/1/1/3 | 5 | 5 |
+| [Overview](Overview/findings.md) | 2026-11-09 | `91fb2b4` | 0/0/1/3 | 4 | 5 |
 | [Secrets](Secrets/findings.md) | 2026-11-09 | `91fb2b4` | 0/0/6/13 | 19 | 19 |
 | [Telemetry](Telemetry/findings.md) | 2026-11-09 | `91fb2b4` | 0/0/0/3 | 3 | 11 |
 | [Theme](Theme/findings.md) | 2026-11-09 | `91fb2b4` | 0/0/1/1 | 2 | 8 |
@@ -80,11 +80,9 @@ description, location, recommendation — lives in the library's `findings.md`.
 
 _None open._
 
-### High (1)
+### High (0)
 
-| ID | Library | Title |
-|----|---------|-------|
-| Overview-001 | [Overview](Overview/findings.md) | Any non-`HttpRequestException` from one instance faults the poller and stops the whole dashboard |
+_None open._
 
 ### Medium (15)