Unify panel queries behind a single Prometheus-compatible datasource

[sudosalim]

The plugin previously spoke to three datasources (Prometheus, a forked couchbase-datasource, and a ProxyPrometheus reverse proxy for snapshot comparison), with panel-building code branching per datasource and a
PromQL→SQL++ translator living inside the plugin backend. This collapses all of that into one datasource served by a standalone sidecar: the frontend always speaks PromQL, and the gateway decides where the data actually comes from.
The plugin backend no longer participates in querying at all, it manages settings, snapshot metadata, and datasource reconciliation.

New service: datasource-gateway/

A standalone Go service exposing a strict superset of the Prometheus HTTP API. Per query it routes by
snapshot:

• Prometheus-backed: PromQL passes straight through to Prometheus/Mimir via a streaming reverse proxy.
• Couchbase-backed: PromQL is translated to SQL++ and evaluated by the real Prometheus engine over the fetched samples, so rate/irate/increase and every other construct match a passthrough by construction.
• Overlap comparison (job=~"a|b"): fans out per snapshot, queries each over its own window, and shifts timestamps to a shared t=0 axis so runs of different lengths can be overlaid.

Because it's API-compatible, a deployment that needs neither Couchbase nor overlap can point straight at Prometheus and skip the gateway entirely.

Routing is driven by the snapshot's metadata document:

Field	Meaning
ts_start	Start of the snapshot window.
ts_end	End of the window. The sentinel "now" (or absent) marks a still-running snapshot, whose window ends at the current time and is re-resolved rather than cached.
store	"couchbase" routes through the SQL++ path; anything else, including absent, serves from Prometheus.

Metadata is resolved once per snapshot and cached (with TTL), with concurrent lookups for the same snapshot sharing a single fetch.

Plugin and frontend

• Panels are PromQL-only; the per-panel datasource field and the DataSourceToggle UI are gone, as is utils.cbquery.
• Overlap comparison uses the same datasource as everything else. The gateway does the time alignment server-side.
• New datasourceCapabilities service reads /config/datasources and gates gateway-only affordances (currently the overlap toggle), degrading closed if the endpoint is unavailable.
• New gateway block in app settings (enable / URL / overlap), configurable from the Configuration page or provisioning.
• The reconciler is the single writer of datasources. It owns the prometheus UID and deletes any pre-existing cbdatasource, so existing deployments clean themselves up on the first reconcile pass.

Configuration and deployment

The gateway is configured by layering, in increasing precedence: built-in defaults → configs/datasource-gateway/config.yaml → DSG_* environment variables → section.field=value arguments. Secrets should go through the environment; an argument would be visible via ps/docker inspect.

The gateway is the generalisation of the https://github.com/m-tarhon/SyncedApp  implementation.