Feat/v0.3 upsert stores

[Achal13jain]

Summary

Adds v0.3.0 support for safer ingestion workflows with store-backed sidecars and a Python safe_upsert() API.

This PR adds:

• safe_upsert() for validating, fixing, storing sidecar payloads, and then upserting cleaned records.
• File and SQLite sidecar stores.
• Content-addressed sidecar references to reduce duplicated payload storage.
• --sidecar-store file|sqlite support for fix and hydrate.
• Store-backed hydration and sidecar migration helpers.
• Updated docs, changelog, README, and landing page for v0.3.0.

Testing

python -m pytest
ruff check .
ruff format --check .
mypy vectormeta
python -m build --outdir C:\tmp\vectormeta-build-v0.3.0

Checklist

• Core logic remains independent from Typer/Rich where practical.
• New behavior has focused tests.
• Docs or examples are updated when user-facing behavior changes.
• Vector database limit claims are cautious and sourced where needed.