chore(signals): throttle report content edits

Problem

signals_reports_partial_update (partial_update in products/signals/backend/views.py) lets a project member edit a report's title and summary. It has no throttle of its own, and session-authenticated requests do not hit the default API-key throttles.

Each edit that actually changes a value writes to the report and, since #73704, emits one document_embeddings request per configured model. So a caller alternating between two values can drive embedding-worker and provider load in a loop.

Two things bound this today, which is why it is low rather than urgent:

• The endpoint already no-ops when a value is unchanged (if "title" in data and data["title"] != report.title), so a plain replay of the same payload costs nothing and never reaches a save.
• The per-edit embedding cost is not new. Before #73704's retraction work an edit emitted a live embedding of the new text; afterwards it emits a tombstone instead. Same number of requests either way.

Scope

Add throttling appropriate to a user-facing write endpoint, scoped per user or per team rather than per API key so session auth is covered. Worth checking whether other report write actions on the same viewset (state, bulk-state) want the same treatment, since they are equally session-reachable.

A cheaper complementary option: coalesce rapid successive edits to the same report so a burst produces one embedding write rather than one per keystroke-level save.

Context

Raised by automated review on #73704. Left out of that PR because it is a property of the edit endpoint rather than of what gets indexed, and belongs with the endpoint's owner.

Start in products/signals/backend/views.py at signals_reports_partial_update and inspect how report write actions are throttled, including state and bulk-state. Determine whether throttling or coalescing best fits the endpoint owner and whether related actions need the same protection. Done means session-authenticated edits are bounded per user or team without changing the existing no-op behavior.