ref(store): Restructure Store Module

[george-sentry]

Linear

Completes STREAM-875

Description

The store module is not organized very well right now. It's basically centered around one large file, inflight_activation.rs, which mixes together types representing activations, shared helper types, the main store trait, and the SQLite adapter implementation. Meanwhile, the Postgres implementation lives separately in posgres_activation_store.rs.

We need a new layout that makes responsibilities explicit. There should be separate files for core activation types, shared supporting types, traits, adapters, and tests.

Changes

• Move store trait into store/traits.rs (plural because I plan to break this apart into smaller traits in the future)
• Move activation and activation status types into store/activation.rs
• Move shared supporting types like BucketRange and DepthCounts into store/types.rs
• Move Postgres and SQLite implementations into store/adapters module
• Keep tests in store/tests.rs
• Update all imports across the codebase to reference these changes

Note that there should be no changes to behavior. This is mostly just moving code around.

[linear-code]

STREAM-875 Restructure Store Module

[markstory]

Looking good