feat(164): guided remediation for health checks (server-declared {Kind, Target})

Make the ~14 health checks actionable: each check that has a fix now declares
where to go, and the SPA acts on it.

Backend:
- Widen domain HealthCheckLink (string Link) -> (string Target, HealthCheckLinkKind
  Kind) with ExternalDoc|AppRoute + factories; only the 4 link-building checks and
  the API mapper touched .Link.
- Evolve HealthCheckResponseModel additively (/api/v1 frozen-additive): keep
  deprecated string? Link (still populated), add Brief (the BriefMessage the mapper
  was silently dropping) and nested Remediation {Kind, Target}. Kind is a mapped
  string, not a wire enum.
- Make Mapper.GetStatus total: NotApplicable no longer throws (defensive; handler
  still filters it). InternalsVisibleTo(ErsatzTV.Tests) added to unit-test totality.
- Fix 2 stale Blazor route links (media/trash -> /app/trash, search?query ->
  /app/search); add AppRoute remediation to actionable checks that had none
  (libraries / schedules / ffmpeg-profiles / settings).

SPA:
- DashboardScreen health panel renders remediation: AppRoute -> client-side nav
  button, ExternalDoc -> new-tab anchor; detail text truncates with title-hover.
- Remove the dead "Open Classic UI" -> /system/health row from SettingsScreen
  (a #91b leftover that just 302'd to /app); update its regression test.

Docs: decisions.md (#164), api-conventions.md (deprecate-in-place DTO evolution),
blazor-route-parity.md (Section 4 correction); v1.json/v1.d.ts/endpoint-index
regenerated.

fixes #164

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/shell.css

@@ -552,6 +552,29 @@ body {
   border-top: 0;
 }
 
+.ctv-health-detail {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: var(--space-4, 8px);
+  min-width: 0;
+}
+
+/* the detail text lives in this inner span now that the cell is a flex wrapper; the
+   .ctv-health-row > span:nth-child(3) rule can no longer truncate it (flex box), so restore
+   the ellipsis affordance here on the actual text element. */
+.ctv-health-detail > span:first-child {
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.ctv-health-action {
+  flex: none;
+  white-space: nowrap;
+}
+
 .ctv-health-icon {
   width: 24px;
   height: 24px;