chore(deps): update go toolchain directive to v1.26.3

See merge request !271

chore(deps): update golang docker tag to v1.26.3

See merge request !272

The backfill heartbeat (readview_status.updated) was refreshed only at
batch commits in doBackFill — once every backFillBatchSize events
(default 10). A single slow EventHandler call could exceed the
heartbeat window (default 5s), at which point backFillInProgress would
consider the filler dead and another instance would start a parallel
backfill against the same read model, double-applying every event still
in flight.

Run the heartbeat in its own errgroup goroutine alongside doBackFill
and the event producer. Ticks at heartbeat/2 regardless of what the
backfill body is currently doing, exits cleanly when doBackFill signals
done via a defer-close, returns nil on ctx cancel so errgroup does not
treat shutdown as a failure, and tolerates individual write errors
(logged at warn level) so a transient DB hiccup does not abort the
backfill.

Tests cover periodic firing, ctx cancellation, the zero-interval
short-circuit, and graceful handling of write failures.

fix(readview): refresh backfill heartbeat from its own goroutine

See merge request !273

chore(release): prepare for v0.6.10

See merge request !274