ragleap-rag (Java) v0.8.0

Adds WeaviateBackend — a fourth vector backend alongside pgvector, FaissBackend, and QdrantBackend. Talks to Weaviate's REST + GraphQL API directly (no gRPC client, no new Maven dependency), with deterministic object IDs, a SQLite text sidecar, self-provided vectors, and a cosine-distance-to-similarity conversion confirmed live before any code was written. 17 new tests, all live against a real running Weaviate instance — the first live-verified Weaviate backend across either language for this project (the Python WeaviateBackend's own source documents that no Weaviate instance was ever available to test it against). Published to Maven Central: io.github.antonyrag:ragleap-rag:0.8.0