knowledge: fact supersession + photo-date valid_from

Two Phase-2 followups in one commit since they're coupled at the
write path:

* Agent populates valid_from from the source photo's date_taken
  when calling store_fact. Loose semantics — date_taken is *evidence
  at that date*, not strictly when the fact started being true — but
  gives the curator a calendar anchor and pairs with supersession to
  close intervals cleanly. valid_until stays NULL (a single photo
  can't tell us when something stopped). Honours the existing
  upsert_fact dedup (corroborated facts keep their first-recorded
  valid_from).

* Supersession: new column entity_facts.superseded_by INTEGER
  (migration 2026-05-10-000200), new status value 'superseded',
  new DAO method supersede_fact, new HTTP endpoint
  POST /knowledge/facts/{id}/supersede.

  Marking an old fact as replaced by a new one atomically: flips
  status to 'superseded', sets superseded_by, and stamps
  valid_until from the new fact's valid_from (when not already
  set). delete_fact clears dangling supersession pointers in the
  same transaction so the column never points at a missing row —
  no FK because SQLite can't ALTER ADD with REFERENCES, but the
  DAO maintains the invariant.

Pairs with conflict detection from the previous slice: once the
old fact's valid_until is closed, its interval no longer overlaps
the new fact's, so they stop flagging — the supersede action
resolves the conflict.

Two tests pin the contract: supersede stamps valid_until from
new.valid_from while respecting an existing valid_until, and
deleting the supersedeR clears the dangling pointer while leaving
the old fact's 'superseded' status in place for history.

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

src/ai/insight_generator.rs

@@ -2672,6 +2672,19 @@ Return ONLY the summary, nothing else."#,
             file_path
         );
 
+        // Anchor the fact in valid-time using the source photo's
+        // `date_taken` (Apollo's naive-as-UTC convention is fine
+        // here — we only care about calendar ordering, not absolute
+        // UTC). The semantic stretch: a photo *evidences* the fact at
+        // that date — the fact may have started earlier — so this is
+        // best read as "no later than this it started being true",
+        // not a strict lower bound. Still useful: gives the curator a
+        // calendar anchor and lets supersession (next slice) close
+        // intervals cleanly when a newer fact arrives. valid_until
+        // stays NULL — a single photo can't tell us when something
+        // *stopped* being true.
+        let valid_from = self.fetch_exif(file_path).and_then(|e| e.date_taken);
+
         let fact = InsertEntityFact {
             subject_entity_id,
             predicate,
@@ -2684,11 +2697,9 @@ Return ONLY the summary, nothing else."#,
             created_at: chrono::Utc::now().timestamp(),
             persona_id: persona_id.to_string(),
             user_id,
-            // The agentic loop doesn't yet derive valid-time from the
-            // photo's date_taken. Left NULL for now; Phase 2's
-            // supersession + a future agent tool will populate these.
-            valid_from: None,
+            valid_from,
             valid_until: None,
+            superseded_by: None,
         };
 
         let mut kdao = self