fix: detect chart SQL dialect from queue connection

[vishaltps]

Summary

• Fixes the activity chart 500ing the dashboard on multi-database setups where Solid Queue runs on a different database engine than the host app's primary connection.
• Detects the SQL dialect from the Solid Queue model's own connection instead of ActiveRecord::Base.

Problem

On a multi-database app with a MySQL primary and a dedicated PostgreSQL queue database (config.solid_queue.connects_to = { database: { writing: :queue } }), the chart's time-bucketing query raised:

PG::UndefinedFunction: ERROR: function unix_timestamp(timestamp without time zone) does not exist
SELECT FLOOR((UNIX_TIMESTAMP(created_at) - ...) / 3600) ...

ChartDataService#adapter? picked the SQL dialect from ActiveRecord::Base.connection — the primary (MySQL) database — so it emitted MySQL UNIX_TIMESTAMP() SQL and then executed it against the Postgres queue database. Because show_chart defaults to true and the chart data is computed inline in OverviewController#index with no rescue, this took down the entire dashboard page for affected users, not just the chart.

Fixes #42.

Solution

Detect the adapter from the model's own connection (SolidQueue::Job.connection), which is the connection the query actually runs on. The model is threaded through bucket_index_expr into adapter?.

For single-database apps and same-engine multi-database apps, SolidQueue::Job.connection resolves to the same adapter as ActiveRecord::Base.connection, so the generated SQL is byte-identical — no behavioral change. Only the previously-crashing mixed-engine path changes.

Changes

• app/services/solid_queue_monitor/chart_data_service.rb: adapter?(model, name) reads model.connection.adapter_name; bucket_index_expr takes the model.
• spec/services/solid_queue_monitor/chart_data_service_spec.rb: new adapter-detection specs per engine (Postgres / MySQL / Trilogy / SQLite) plus a regression case reproducing config.show_chart = true fails with postgres #42 (MySQL primary + Postgres queue model → asserts EXTRACT(EPOCH ...), never UNIX_TIMESTAMP).
• CHANGELOG.md: [Unreleased] → Fixed entry.

Testing

• Unit tests added (adapter detection + config.show_chart = true fails with postgres #42 regression)
• Full suite green: 346 examples, 0 failures
• bundle exec rubocop clean on changed files
• spec/requests/solid_queue_monitor/csp_compatibility_spec.rb passes
• Edge cases considered (single-DB and same-engine multi-DB produce identical SQL)

Backward compatibility

Fully backward compatible. Safe to ship as a patch release (2.2.1) — no config, no migration.

Checklist

• Code follows project patterns
• Self-review completed
• Tests pass
• No debug statements left