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

[kitalkuyo-gita]

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