feat: add priority lanes to worker scheduler

[brodapeethar]

Summary

Add three weighted priority lanes (High / Normal / Low) to the worker scheduler so that urgent webhook deliveries preempt background cleanup tasks. The scheduler uses weighted round-robin to select the next job, with a starvation guard that forces a low-lane pick after too many consecutive high/normal picks.

Changes

File	Change
internal/worker/job.go	Add Priority type (High / Normal / Low), Priority field on Job, DefaultLaneWeights (3:2:1), LaneDepth on JobStore interface, SortJobs helper
internal/worker/scheduler.go	Rewrite: Scheduler.Next() implements weighted round-robin across lanes, SetWeights() and SetStarvationLimit() for configuration, all Schedule* methods now accept a Priority argument
internal/worker/store_memory.go	ListPending sorts by priority then time, new ListPendingByPriority and LaneDepth methods, copyJob helper
internal/worker/worker.go	pollAndDispatch uses scheduler.Next() instead of store.ListPending(), emits LaneDepth and LanePickedTotal metrics, LaneWeights in Config
internal/worker/scheduler_test.go	14 new tests (RR distribution, starvation guard, empty-lane fallback, custom weights, lane depth, concurrent safety)
QUICK_START.md, README.md, TEST_EXECUTION.md	Update examples to include Priority parameter

Design

Weighted round-robin

• High weight 3, Normal weight 2, Low weight 1 → cycle H H H N N L
• If the chosen lane is empty, fall back to strict priority order
• Weights are config-driven via Config.LaneWeights or scheduler.SetWeights()

Starvation guard

• After starvationLimit (default 10) consecutive high/normal picks, the next pick forces the low lane
• Configurable via scheduler.SetStarvationLimit()

Metrics

• LaneDepth[Priority] — number of pending jobs per lane
• LanePickedTotal[Priority] — cumulative picks per lane

Overload protection

• Existing MaxQueueDepth backpressure still gates the poll loop before any lane selection

Test output

go test -count=1 -v ./internal/worker/...
=== RUN   TestScheduler_ScheduleCharge          --- PASS
=== RUN   TestScheduler_ScheduleInvoice         --- PASS
=== RUN   TestScheduler_ScheduleReminder        --- PASS
=== RUN   TestMemoryStore_SortByPriorityThenTime --- PASS
=== RUN   TestScheduler_WeightedRoundRobin      --- PASS (50% high, 33% normal, 17% low)
=== RUN   TestScheduler_StarvationGuard         --- PASS
=== RUN   TestScheduler_EmptyLanesFallback      --- PASS
=== RUN   TestScheduler_NextReturnsNilWhenEmpty --- PASS
=== RUN   TestMemoryStore_LaneDepth             --- PASS
=== RUN   TestScheduler_FallsBackToStrictPriority --- PASS
=== RUN   TestWorkerMetrics_LaneDepth           --- PASS
=== RUN   TestScheduler_CustomWeights           --- PASS
=== RUN   TestScheduler_ConcurrentSafe          --- PASS
PASS (28 total tests)

Closes Stellabill#329