feat: add entity-relationship knowledge memory to agentic insights

Implements persistent cross-photo knowledge memory so the agentic
insight loop can learn and recall facts about people, places, and
events across the photo collection.

Changes:
- photo_insights: drop UNIQUE(file_path) + INSERT OR REPLACE, replace
  with append-only rows + is_current flag for insight history retention
- New tables: entities, entity_facts, entity_photo_links with FK
  constraints and confidence scoring
- KnowledgeDao trait + SqliteKnowledgeDao with upsert, merge, and
  corroboration (confidence +0.1 on duplicate fact detection)
- Four new agent tools: recall_entities, recall_facts_for_photo,
  store_entity, store_fact (with object_entity_id FK support)
- Cameron entity auto-seeded with stable ID injected into system prompt
- Pre-run photo link clearing + post-loop source_insight_id backfill
- Audit REST API: GET/PATCH/DELETE /knowledge/entities/{id},
  POST /knowledge/entities/merge, GET/PATCH/DELETE /knowledge/facts/{id},
  GET /knowledge/recent

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/database/models.rs

@@ -1,4 +1,7 @@
-use crate::database::schema::{favorites, image_exif, photo_insights, users, video_preview_clips};
+use crate::database::schema::{
+    entities, entity_facts, entity_photo_links, favorites, image_exif, photo_insights, users,
+    video_preview_clips,
+};
 use serde::Serialize;
 
 #[derive(Insertable)]
@@ -82,6 +85,7 @@ pub struct InsertPhotoInsight {
     pub summary: String,
     pub generated_at: i64,
     pub model_version: String,
+    pub is_current: bool,
 }
 
 #[derive(Serialize, Queryable, Clone, Debug)]
@@ -92,6 +96,79 @@ pub struct PhotoInsight {
     pub summary: String,
     pub generated_at: i64,
     pub model_version: String,
+    pub is_current: bool,
+}
+
+// --- Knowledge memory models ---
+
+#[derive(Insertable)]
+#[diesel(table_name = entities)]
+pub struct InsertEntity {
+    pub name: String,
+    pub entity_type: String,
+    pub description: String,
+    pub embedding: Option<Vec<u8>>,
+    pub confidence: f32,
+    pub status: String,
+    pub created_at: i64,
+    pub updated_at: i64,
+}
+
+#[derive(Serialize, Queryable, Clone, Debug)]
+pub struct Entity {
+    pub id: i32,
+    pub name: String,
+    pub entity_type: String,
+    pub description: String,
+    pub embedding: Option<Vec<u8>>,
+    pub confidence: f32,
+    pub status: String,
+    pub created_at: i64,
+    pub updated_at: i64,
+}
+
+#[derive(Insertable)]
+#[diesel(table_name = entity_facts)]
+pub struct InsertEntityFact {
+    pub subject_entity_id: i32,
+    pub predicate: String,
+    pub object_entity_id: Option<i32>,
+    pub object_value: Option<String>,
+    pub source_photo: Option<String>,
+    pub source_insight_id: Option<i32>,
+    pub confidence: f32,
+    pub status: String,
+    pub created_at: i64,
+}
+
+#[derive(Serialize, Queryable, Clone, Debug)]
+pub struct EntityFact {
+    pub id: i32,
+    pub subject_entity_id: i32,
+    pub predicate: String,
+    pub object_entity_id: Option<i32>,
+    pub object_value: Option<String>,
+    pub source_photo: Option<String>,
+    pub source_insight_id: Option<i32>,
+    pub confidence: f32,
+    pub status: String,
+    pub created_at: i64,
+}
+
+#[derive(Insertable)]
+#[diesel(table_name = entity_photo_links)]
+pub struct InsertEntityPhotoLink {
+    pub entity_id: i32,
+    pub file_path: String,
+    pub role: String,
+}
+
+#[derive(Serialize, Queryable, Clone, Debug)]
+pub struct EntityPhotoLink {
+    pub id: i32,
+    pub entity_id: i32,
+    pub file_path: String,
+    pub role: String,
 }
 
 #[derive(Insertable)]