Define update behavior for warehouse person and group property mappings

Feature request

Is your feature request related to a problem?

Warehouse-backed person and group property sources accept column_property_map and column_descriptions on create, but the update API does not accept those fields. Correcting a mapping therefore requires deleting and recreating the source and changes its identity.

Describe the solution you'd like

Allow PATCH to update column_property_map and column_descriptions for existing warehouse-backed person and group property sources.

The focused change in #89260:

• applies the existing create-time validation and normalization
• requires editor access to the bound warehouse object when profile mapping fields are supplied
• preserves the source identity and saves the update under the existing row lock
• requests the existing binding-level backfill when an enabled source's effective mapping changes
• updates descriptions on existing property definitions without requiring a value backfill
• keeps profile sources on the warehouse path and account sources on the account path
• accepts null profile-only fields when an account-source GET representation is sent back through PATCH
• avoids scheduling work for an effective no-op

Describe alternatives you've considered

Deleting and recreating the source loses its identity. Updating database fields out of band is not a supported API contract.

Additional context

Done means #89260 delivers the PATCH contract and focused regression coverage listed above.

This issue does not redesign ordering between incremental syncs and backfills, retries after activity failure, property-definition creation or provenance ownership, the UI editor, or MCP instructions. It uses the existing synchronization mechanisms rather than defining a new end-to-end orchestration contract.

Debug info

- [ ] PostHog Cloud, region and project ID: not applicable; source-level API contract gap
- [x] PostHog Hobby self-hosted with `docker compose`, version/commit: current source and implementation are tracked in #89260
- [ ] PostHog self-hosted with Kubernetes (deprecated, see `Sunsetting Kubernetes support`), version/commit: not applicable

Start with the focused change in #89260 and its regression coverage; the issue does not name implementation files or test paths. Done means the PATCH contract and the listed validation, access, locking, backfill, description, source-path, null-field, and no-op behaviors are covered.