[geaflow/ai-memory] Define `GraphBackend` SPI

Priority: P0
Difficulty: Advanced

Context: Current main path uses in-process `MemoryGraph`; `GraphComputeEngine` is an empty interface. Phase 1 needs a backend boundary so local persistence and a GeaFlow vertical slice can share contracts.

Scope:

- Define `GraphBackend` interface for upsert vertex, upsert edge, delete, scan, get schema, and transaction boundary.
- Define capability flags: local, persistent, distributed, supports checkpoint, supports tenant scope.
- Add contract tests using an in-memory fake backend.

Constraints:

- Do not replace `MemoryGraph` everywhere in this issue.
- Do not claim distributed production capability.
- Public SPI needs maintainer review.

Acceptance Criteria:

- A fake backend passes contract tests.
- Existing `GraphAccessor` integration path can be adapted later without breaking tests.

Suggested paths:

- `geaflow-ai/src/main/java/org/apache/geaflow/ai/backend`

Start in geaflow-ai/src/main/java/org/apache/geaflow/ai/backend and read the empty GraphComputeEngine interface alongside the current in-process MemoryGraph path. Define the GraphBackend contract and capability flags, then add contract tests with an in-memory fake backend. Done means the fake passes the contracts while existing GraphAccessor integration remains unchanged.

Written by the indexing model from the issue text.