feature: add arbitrary metadata along with vectors

[mmailhos]

This PR adds support for metadata alongside vectors. It helps store arbitrary data (tags, K:V pairs) along with the indexable text.

[mmailhos]

This PR also introduces a breaking change: the POST /vector is no longer supported.
Reasons:

1. Consistency: All embeddings are generated by the same model and normalization pipeline, ensuring vectors live in a coherent semantic space.
2. Simplicity: Clients only send text — no need to manage model versions, dimensions, or preprocessing — making integration trivial and error-free. (DX improvement)
3. Security: Prevents injection of arbitrary or adversarial vectors that could corrupt search results or break similarity guarantees.

[mmailhos]

Side note: I'll also be looking at some memory optimisations in sub-sequent PRs (prevent keeping multiple copies, or preventing repeated cloning).