personas: composite FK + built-in update guard

Two persona-infrastructure correctness fixes that go together because
the second one (FK with CASCADE) requires the first (preventing the
persona row from being mutated out from under its facts).

1. update_persona handler refuses name/systemPrompt edits to built-ins
   (409). includeAllMemories stays editable — that's a per-user
   preference, not the persona's identity. Mirrors the existing
   delete_persona guard. The DAO is intentionally permissive so the
   guard sits at the HTTP layer; persona_dao test pins that contract.

2. Migration 2026-05-10 adds user_id to entity_facts and a composite
   FK (user_id, persona_id) -> personas(user_id, persona_id) ON DELETE
   CASCADE. This closes two issues at once:

   - Persona orphans: deleting a custom persona used to leave its
     facts dangling forever, readable only via PersonaFilter::All.
     CASCADE now wipes them with the persona row.

   - Multi-user fact leakage: PersonaFilter::Single("default") used
     to surface every user's default-scoped facts. PersonaFilter is
     now { user_id, persona_id } and all read paths
     (get_facts_for_entity, list_facts, get_recent_activity) filter
     on user_id first. upsert_fact's dedup key extends to user_id so
     identical claims under shared persona names from different
     users no longer corroborate-bump each other's confidence.

   - user_id threads from Claims.sub.parse::<i32>().unwrap_or(1) at
     the chat / insight handlers through ChatTurnRequest, the
     streaming agentic loop, execute_tool, and into the leaf tools
     (tool_store_fact, tool_recall_facts_for_photo). The ".unwrap_or(1)"
     accommodates Apollo's service token whose sub is non-numeric on
     legacy mints.

   - Backfill picks the smallest user_id matching each legacy fact's
     persona_id so the FK holds for already-stored rows.

Five new knowledge_dao tests with FK-on connection: persona scoping
isolation, All-variant union per-user, dedup not crossing users,
CASCADE delete, FK rejection of unknown personas. Plus
dao_update_does_not_block_built_ins documenting where the
HTTP-layer guard lives.

Apollo coordinates separately — the matching changes there add the
/api/personas proxy and start sending persona_id on photo-chat turns.

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

src/personas.rs

@@ -216,6 +216,35 @@ async fn update_persona(
     let cx = opentelemetry::Context::current();
     let mut dao = dao.lock().expect("Unable to lock PersonaDao");
 
+    // Built-in personas are owned by the migration; the canonical voice
+    // text lives in source. A client renaming or rewriting the prompt
+    // here would diverge from what new users get seeded with and hide
+    // the operator's actual customization (their own custom persona)
+    // from the picker. `include_all_memories` stays editable on
+    // built-ins — that's a per-user preference, not the persona's
+    // identity. Mirrors the same guard delete_persona enforces below.
+    match dao.get_persona(&cx, uid, &pid) {
+        Ok(Some(p)) if p.is_built_in => {
+            let editing_identity =
+                body.name.is_some() || body.system_prompt.is_some();
+            if editing_identity {
+                return HttpResponse::Conflict().json(serde_json::json!({
+                    "error": "Cannot edit name or systemPrompt of a built-in persona"
+                }));
+            }
+        }
+        Ok(None) => {
+            return HttpResponse::NotFound()
+                .json(serde_json::json!({"error": "Persona not found"}));
+        }
+        Err(e) => {
+            log::error!("update_persona lookup error: {:?}", e);
+            return HttpResponse::InternalServerError()
+                .json(serde_json::json!({"error": "Database error"}));
+        }
+        Ok(Some(_)) => {}
+    }
+
     let patch = PersonaPatch {
         name: body.name.clone(),
         system_prompt: body.system_prompt.clone(),