Detect outdated Helm charts per cluster; return 503 for unreachable MCP endpoints

[gianlucam76]

Outdated Helm chart detection

• /helmcharts responses now include, per release, the latest upstream version and latest same-minor patch version when a newer one is published. Sourced from addon-controller's periodic upstream check, so the dashboard can show "update available" without a separate lookup.
• /capiclusters and /sveltosclusters responses now include a hasOutdatedHelmCharts flag per cluster, so clusters with pending chart updates can be flagged in list views the same way hasIssues already flags clusters with failures.
• Reconciliation now reacts specifically to changes in that upstream-check status, while filtering out unrelated status churn (e.g. next-reconcile-time ticking) that previously could trigger reconciles without any user-visible effect.

MCP server availability

• Endpoints that depend on mcp-server (installation check, profile/cluster deployment debugging, event/classifier pipeline analysis) now return 503 Service Unavailable with a clear message when mcp-server isn't deployed or unreachable, instead of an opaque 500.
• Documented these MCP-backed endpoints in the README — they existed already but had never been written up.

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