feat(embed): add embedding job drain

[GaosCode]

Summary

This PR is logically stacked on #32, but it targets main because this is a fork-based contribution.

It adds the storage and execution layer for queued message embeddings. sync --with-embeddings queues embedding work without calling the provider, and the new discrawl embed command explicitly drains pending jobs in bounded batches.

Until #32 lands, this PR will also show the provider abstraction diff from that PR. Please review #32 first. After #32 is merged, this PR should collapse to the embedding job drain changes only. If the upstream merge strategy prevents that, I will rebase this branch.

Changes

• Add schema version 2 migration for embedding job metadata and stored message vectors.
• Add local message_embeddings storage keyed by message, provider, model, and input version.
• Extend embedding_jobs with provider/model/input version, error tracking, and lock metadata.
• Add discrawl embed with --limit, --batch-size, and --rebuild.
• Drain pending embedding jobs through the configured provider with capped input text.
• Keep sync --with-embeddings non-blocking by only queueing work.
• Avoid requeueing completed embedding jobs when unchanged messages are synced again.
• Treat provider HTTP 429 responses as rate limits and stop the current drain run cleanly.

Testing

• go test ./...

Added coverage for:

• migrating unversioned v1 databases to schema v2
• preserving completed embedding jobs across unchanged syncs
• requeueing embedding work when normalized message content changes
• draining pending jobs into stored vectors
• handling empty content, provider failures, rebuilds, and rate limits

Risks / Notes

• This PR does not add semantic or hybrid search ranking yet. It only adds the local vector storage and explicit embedding queue drain needed by the follow-up search PR.
• Documentation updates are intentionally left out of this PR and can be submitted separately.
• Because this PR targets main while depending on #<PR2_NUMBER>, the initial diff may be noisy until feat(embed): add local embedding providers #32 is merged.