fix(db): error on embedding dimension mismatch

[echobt]

When the embedding model changes after indexing, stored chunk embeddings can have a different dimensionality than the query embedding. Previously this was silently treated as 0.0 similarity, producing confusing 0% scores.

This change detects dimension mismatches (and corrupt embedding blobs) during search and returns a clear error suggesting reindexing via: vgrep index --force.

A unit test covers the mismatch case.

Validation:

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test