StorageService: Add missing methods for InMemory provider

Summary

The InMemory storage backend (asab/storage/inmemory.py) does not fully implement the StorageServiceABC interface. MongoDB is the reference implementation; InMemory is missing several features that application code may rely on.

Feature parity

Method / feature	MongoDB	InMemory	ElasticSearch
get_by()	✅	❌	❌
generate_id()	✅	✅	❌
list()	✅	❌	✅

InMemory gaps (this issue):

Method / feature	Status
get_by()	❌ raises NotImplementedError()
list()	❌ not implemented

Background

InMemory is used for testing and lightweight deployments. Upsertor modifiers ($set, $inc, $push, $unset, $pull) and AES encrypt/decrypt already work. The main gaps are lookup by field, changestream webhooks, and list().

Current state

• get_by() — asab/storage/inmemory.py:123–130 raises NotImplementedError()
• list() — not present on InMemory StorageService

Proposed work

• Implement get_by() (scan in-memory collection by key/value; raise KeyError("NOT-FOUND") when missing, matching MongoDB)
• Implement list() for basic pagination/sorting parity with MongoDB

References

• Backend: asab/storage/inmemory.py
• Reference: asab/storage/mongodb.py
• Webhook base: asab/storage/upsertor.py
• Config: [asab:storage:changestream] (webhook_uri, webhook_auth)

Acceptance criteria

• get_by() returns a document by key/value or raises KeyError("NOT-FOUND")
• Tests cover get_by() and webhook emission