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/database/models.rs

@@ -243,6 +243,12 @@ pub struct InsertEntityFact {
     /// real-world referents. Defaults to `'default'` for legacy rows
     /// (see migration 2026-05-09-000000).
     pub persona_id: String,
+    /// Author's user_id. Required for the composite FK to
+    /// `personas(user_id, persona_id)` (migration 2026-05-10-000000) and
+    /// for cross-user fact isolation: two users with the same 'default'
+    /// persona must not see each other's facts. Always paired with
+    /// `persona_id` — they're a unit.
+    pub user_id: i32,
 }
 
 #[derive(Serialize, Queryable, Clone, Debug)]
@@ -258,6 +264,7 @@ pub struct EntityFact {
     pub status: String,
     pub created_at: i64,
     pub persona_id: String,
+    pub user_id: i32,
 }
 
 #[derive(Insertable)]