Schedule plugin background tasks with TickerQ

[PianoNic]

Summary

• Replaced the hand-rolled plugin task loop (PluginHost.RunTaskLoop plus the in-memory PluginSchedulerRegistry) with TickerQ, persisted to SchulyDbContext through the EF Core operational store. Schedules, next-run times and run history now survive a restart, cron replaces drifting intervals, and failed runs retry per the plugin's declared policy.
• Resolved the source-generator vs runtime-plugin problem the issue calls out: TickerQ discovers ticker functions with a compile-time source generator, so a [TickerFunction] inside a plugin assembly loaded into a collectible AssemblyLoadContext could never be seen. The host instead owns a single [TickerFunction("RunPluginTask")] that carries { plugin, task } and dispatches into the plugin's own DI scope by name. Plugins never reference TickerQ.
• On load, each IPluginBackgroundTask is upserted as a cron ticker keyed "<plugin>/<task>", so a reload updates rather than duplicates and a task dropped by a plugin update stops firing; unload withdraws the plugin's tickers and drains in-flight runs before the plugin provider is disposed. RunOnStartup adds a one-shot time ticker.
• Operators can override a task's cadence per deployment in plugins-config/<AssemblyName>.yml under Schedules:<task>:{Cron,Retries,RunOnStartup}; an override that fails validation is ignored in favour of the plugin default rather than taking the task offline.
• GET /api/plugins/scheduler keeps its response shape, now backed by TickerQ's persisted cron tickers and occurrences (last run, next run, last status, failure counts). The TickerQ dashboard is mounted at /tickerq in Development only.
• Each run now executes in a fresh scope on the plugin's provider instead of the plugin root provider, so a scoped SchulyDbContext no longer lives for the process lifetime.
• Migration AddTickerQ creates the ticker schema. Docs updated; PluginBackgroundTaskHost references in docs/architecture.md were stale and now describe the real runtime.

Depends on schulydev/SchulyPluginAbstractions#144, which replaces IPluginBackgroundTask.Interval with PluginSchedule. Version 0.3.0 is not published to nuget.org yet, so CI will fail to restore Schuly.Plugin.Abstractions 0.3.* until that release is cut. It was verified locally against a 0.3.0 pack: dotnet build clean with 0 warnings, dotnet test 52/52 green.

The .gitignore change is not incidental: /src/Schuly.API/plugins/ matched the tracked src/Schuly.API/Plugins/ sources on case-insensitive checkouts, so new files in the plugin runtime folder were silently ignored. The runtime drop folder is now matched by artifact extension instead.

Follow-up, deliberately not in this PR: a host service that lets a plugin trigger a one-shot run of one of its own tasks ("sync this account now") would need a new cross-boundary interface in the abstractions package. The existing on-demand sync mechanism is unchanged here.

Closes #233