Parallel per-type load writes + omnigraph optimize/cleanup CLI (#46)

* Parallel per-type load writes + omnigraph optimize/cleanup CLI

## MR-677.3 — parallel per-type load writes

The load path already groups records into one RecordBatch per type and
makes one Lance commit per table (loader::mod.rs:249-..), but those
commits ran sequentially. Wrap node and edge write loops in
`futures::stream::buffered(N)` against a new helper
`write_batches_concurrently`. Concurrency tunable via
`OMNIGRAPH_LOAD_CONCURRENCY` (default 8).

## MR-676 — `omnigraph optimize` and `omnigraph cleanup`

New CLI subcommands that walk every node + edge table in the repo:

- `omnigraph optimize <uri>` — runs Lance `compact_files` on each
  table to merge small fragments into fewer larger ones.
- `omnigraph cleanup <uri> --keep N | --older-than 7d --confirm` —
  runs Lance `cleanup_old_versions` to prune historical manifests +
  unique fragments. Requires `--confirm` because it's destructive.
  Supports both count-based and time-based retention (or both AND'd
  together). Time uses chrono `DateTime<Utc>` (added as a workspace
  dep, default-features off).

Both commands run their per-table loops in parallel (8-way bounded,
`OMNIGRAPH_MAINTENANCE_CONCURRENCY` env override). Smoke-tested
against the 114-table prod graph: optimize went 7m15s sequential
→ 1m28s parallel. cleanup --keep 1 removed 137 historical versions
across 114 tables in 1m57s without disrupting `/healthz` or query
responses.

Public API on `Omnigraph`:

  pub async fn optimize(&mut self) -> Result<Vec<TableOptimizeStats>>
  pub async fn cleanup(&mut self, opts: CleanupPolicyOptions)
      -> Result<Vec<TableCleanupStats>>

All 10 existing loader tests still pass.

Closes MR-676.
Partially addresses MR-677 (the .3 — parallel by type — piece;
MR-677.1 is for the `omnigraph embed` path, not load, since load
doesn't call Gemini directly. .2 was already in place).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* chore: regenerate openapi.json

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Cargo.lock

@@ -4596,7 +4596,7 @@ dependencies = [
 
 [[package]]
 name = "omnigraph-cli"
-version = "0.3.0"
+version = "0.3.1"
 dependencies = [
  "assert_cmd",
  "clap",
@@ -4616,7 +4616,7 @@ dependencies = [
 
 [[package]]
 name = "omnigraph-compiler"
-version = "0.3.0"
+version = "0.3.1"
 dependencies = [
  "ahash",
  "arrow-array",
@@ -4637,7 +4637,7 @@ dependencies = [
 
 [[package]]
 name = "omnigraph-engine"
-version = "0.3.0"
+version = "0.3.1"
 dependencies = [
  "arrow-array",
  "arrow-cast",
@@ -4646,6 +4646,7 @@ dependencies = [
  "arrow-select",
  "async-trait",
  "base64",
+ "chrono",
  "fail",
  "futures",
  "lance",
@@ -4674,7 +4675,7 @@ dependencies = [
 
 [[package]]
 name = "omnigraph-server"
-version = "0.3.0"
+version = "0.3.1"
 dependencies = [
  "async-trait",
  "aws-config",