refactor(usage): let a store answer precomputed [ING-586]

[vincent-pochet]

Stacked on #6427, which adds the service that builds the bucket set.

Context

A fee computed out of the pre-aggregated ClickHouse buckets is already fresh, so the charge cache
has nothing to add to it: caching it would put back the staleness the buckets remove. Nothing can
express that today, as neither an event store nor the cache knows that some answers need no caching.

Description

• Add precomputed? to the event stores, false everywhere but on the new UsageBucketStore, a
  decorator answering a covered aggregation from a UsageBucketSet and delegating the rest.
• Forward the predicate from an aggregation to its store.
• Take a per-filter bypass on the charge cache middleware, skipping the cache round-trip.
• Hold one event store provider per charge service, passed in by callers running one of their own.
• Reuse a single aggregator per pricing bucket, so the aggregation factory is called once where the
  zero-units hydration used to build a second one for the same bucket.

Nothing mints a UsageBucketStore yet, so precomputed? is false and the bypass is never taken.
The routing that reaches for both is the next PR of the stack.

[lago-claude-ai-agent]

Automated pre-review (advisory, not a required check) — verdict: HOLD · CI green

HOLD — the new precomputed-store path is not connected to production.

• Update Events::Stores::Provider#store_for to return a UsageBucketStore when served_from_buckets?; it currently always returns the raw store, so the new wrapper is unused.
• Pass the aggregator's precomputed? state to ChargeCacheMiddleware#call; no application call supplies the new bypass: argument, so bucket-backed fees would still use the stale fee cache.
• Add an integration spec through the provider and fee service that proves eligible usage comes from buckets and bypasses the fee cache; the new specs exercise the disconnected pieces only in isolation.

[lago-claude-ai-agent]

Automated pre-review (advisory, not a required check) — verdict: PASS · CI green

PASS — The bucket reader/store, provider threading, aggregator reuse, and cache-bypass contract are consistent with the existing aggregation boundaries and have focused coverage for totals, grouping, fallbacks, delegation, and cache behavior.

Non-blocking: the new store spec places several allow calls directly in examples; project test conventions require those stubs in before blocks.

[lago-claude-ai-agent]

Automated pre-review (advisory, not a required check) — verdict: PASS · CI green

PASS — The preparatory store decorator, provider reuse, cache bypass, and aggregator memoization are consistent with the existing interfaces, and the added specs cover their substantive behavior.

Non-blocking: the guard-clause modifiers in grouped_count and grouped_sum conflict with the repository guidance to avoid modifiers immediately before a method's final expression.