Distilled from #9541 to reduce the scope of that PR.
- Part of #9307
- ❗ Blocks #9786
- ❗ Blocks #9541
### Changes 🏗️
- Fix `LibraryAgent` schema (for #9786)
- Fix relationships between `LibraryAgent`, `AgentGraph`, and
`AgentPreset`
- Impose uniqueness constraint on `LibraryAgent`
- Rename things that are called `agent` that actually refer to a
`graph`/`agentGraph`
- Fix singular/plural forms in DB schema
- Simplify reference names of closely related objects (e.g.
`AgentGraph.AgentGraphExecutions` -> `AgentGraph.Executions`)
- Eliminate use of `# type: ignore` in DB statements
- Add `typed` and `typed_cast` utilities to `backend.util.type`
### Checklist 📋
#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] CI static type checking (with all risky `# type: ignore` removed)
- [x] Check that column references in views are updated
353396110c