Store image-index embeddings as fp16

[JPPhoto]

Summary

Persist image and video embeddings as little-endian float16 with explicit per-row encoding. Search and projection continue to use float32. A SQLite migration converts existing embeddings without rerunning the embedding model, and legacy float32 rows remain readable.

Related Issues / Discussions

QA Instructions

• Focused image-index and migration suites: 251 passed.
• Image Map router suite: 93 passed.
• Chromium component tests for Gallery search and Image Map: 41 passed with a mocked API.
• Docs package test: 1 passed. Ruff 0.11.13 lint and format checks passed.
• One-off numerical QC ran against a SQLite backup; the source database was not modified. Across 100 queries against 163,479 images and 23 videos, top-10 overlap was 100%; maximum score drift was 2.0401e-4. There were no ranking changes among 114,421 candidate pairs with an original score gap above 0.002.
• A 163,000-vector migration fixture took 13.31 seconds including backup. Raw embedding BLOB storage halved; a separate VACUUM reduced the fixture database from about 698.9 MB to 361.4 MB.
• Exact Ruff 0.11.2 and the full docs build were unavailable because of DNS failures. The docs build could not fetch the Starlight changelog from GitHub.

Review

No material review findings. A reviewer suggested adding a regression test for replacing existing legacy float32 rows and that can be left as a follow-up if deemed important.

Compatibility / Rollout

The migration runs on first startup after upgrade, in a database transaction, and creates a database backup before conversion. Restore the pre-upgrade backup before running an older application version. SQLite may retain freed pages, so the database file may not shrink immediately.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Meaningful regression coverage added / updated where needed; obsolete tests/code removed
• Persisted-state and API changes include required migrations / compatibility validation
• Relevant performance/efficiency opportunities considered; material claims have evidence
• Material review findings resolved and relevant checks rerun
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)