Trace Pipeline Health

Return queue depth + drop count from the running exporter.

Safe-default: when no exporter is attached to app.state (engine booted without a trace pipeline, or T7 wiring not present), return zeroes plus writer_running=False so the UI panel stays renderable instead of erroring.

Declared before the /{otel_trace_id} route so the literal path takes precedence over the path-parameter route at match time — FastAPI iterates in declaration order.

Also surfaces database_dialect (read off the sessionmaker bind) so the UI can conditionally render the SQLite operational banner without a second round-trip.