Pagination endpoint for Ahnlich DB store

Should probably have a list endpoint that allows iterating through a store. We could provide support for predicate conditions as well which effectively gives us an operation like

```proto
message ListStore {
string store = 1;
optional uint64 start_key = 2; // StoreKeyId perhaps as a token to start from
uint32 limit = 3; // Max number of items to return
optional PredicateCondition condition = 4; // Filter results by predicate
optional string schema = 5; // Schema/namespace
}
```

Given that ordering has no semantic meaning, should we bother to implement ordering. If yes we could still sort by `StoreKeyId` first.

Checklist follow up to this would be:
- Integrating endpoint into Ahnlich AI and CLI
- Updating libraries to include new endpoint
- Updating https://ahnlich.dev public docs
- VectorDBZ as #396 stated

Start with the proposed ListStore proto message and resolve whether pagination, predicate filtering, schema support, and ordering are required. Define the endpoint's completion criteria before implementation, then account for the listed follow-ups: Ahnlich AI and CLI integration, library updates, public documentation, and VectorDBZ support.

Written by the indexing model from the issue text.