Add automatic project snapshots

docs/project-history-and-external-model-architecture.md

@@ -101,6 +101,17 @@ Implemented named-snapshot foundation:
   explicit inline restore confirmation and cursor-based loading of older
   revisions; a successful restore reloads all project read models
 
+Implemented automatic-snapshot policy:
+
+- `project_snapshots.kind` distinguishes `named` from `automatic`
+- the central revision boundary captures the complete post-command state after
+  every 25 additional project revisions
+- the newest 12 automatic snapshots per project are retained
+- retention never removes named snapshots, immutable revisions or physical
+  database backups
+- automatic capture and retention run inside the same transaction as the
+  triggering project revision
+
 ## Revit / CSV / IFCGUID Round-Trip Direction
 
 External model exchange should use explicit staging and link entities instead of writing imported rows directly into circuits.
@@ -258,7 +269,6 @@ Completed foundation:
 
 Remaining constraints before completing project version history:
 
-- define and implement a periodic snapshot and retention policy
 - extend project-scoped commands only when further project mutations are
   deliberately added to history; the generic versioned envelope is complete
 - do not model IFCGUID as an overloaded circuit equipment identifier
@@ -266,7 +276,6 @@ Remaining constraints before completing project version history:
 
 ## Deferred Decisions
 
-- exact snapshot frequency and retention policy
 - whether large snapshot payloads remain in PostgreSQL or move to object storage
 - branch visualization after undo followed by new edits
 - user/role model and actor attribution