Prefs: owner defaults (model, backend, persona_id, reel span/length)

Closes #127.

Grows user_ai_prefs with five nullable owner-default columns and makes
them the server-side fallback layer: request param -> stored pref ->
hard default, at insight generation (agentic + standard), all three
insight-chat turn variants, and reel creation.

- PUT /prefs accepts the fields with the existing three-state partial
  update semantics (TriState generalized to TriState<T> for the numeric
  reel_max_segments); validation (canonical BackendKind::parse,
  MemoriesSpan::from_str) runs before any write.
- GET /prefs returns the new fields.
- capture_prefs reads the existing row and carries the AI owner-defaults
  through unchanged (the upsert overwrites all columns), mirroring only
  the effective reel span/segments.
- MemoriesSpan gains Display + FromStr so stored spans round-trip
  losslessly.

1463/0 tests (+27), clippy clean.

src/database/user_ai_prefs_dao.rs

@@ -100,6 +100,11 @@ impl UserAiPrefsDao for SqliteUserAiPrefsDao {
                     dsl::tz_offset_minutes.eq(&prefs.tz_offset_minutes),
                     dsl::library.eq(&prefs.library),
                     dsl::display_tz.eq(&prefs.display_tz),
+                    dsl::default_model.eq(&prefs.default_model),
+                    dsl::default_backend.eq(&prefs.default_backend),
+                    dsl::default_persona_id.eq(&prefs.default_persona_id),
+                    dsl::reel_span.eq(&prefs.reel_span),
+                    dsl::reel_max_segments.eq(&prefs.reel_max_segments),
                     dsl::updated_at.eq(&prefs.updated_at),
                 ))
                 .execute(connection.deref_mut())
@@ -147,6 +152,11 @@ mod tests {
             library: Some("1".to_string()),
             updated_at: now,
             display_tz: Some("America/New_York".to_string()),
+            default_model: Some("llama3.2:3b".to_string()),
+            default_backend: Some("hybrid".to_string()),
+            default_persona_id: Some("cabin".to_string()),
+            reel_span: Some("week".to_string()),
+            reel_max_segments: Some(24),
         };
         dao.upsert_prefs(&ctx(), &prefs).unwrap();
 
@@ -156,6 +166,11 @@ mod tests {
         assert_eq!(row.tz_offset_minutes, Some(-480));
         assert_eq!(row.library, Some("1".to_string()));
         assert_eq!(row.display_tz, Some("America/New_York".to_string()));
+        assert_eq!(row.default_model, Some("llama3.2:3b".to_string()));
+        assert_eq!(row.default_backend, Some("hybrid".to_string()));
+        assert_eq!(row.default_persona_id, Some("cabin".to_string()));
+        assert_eq!(row.reel_span, Some("week".to_string()));
+        assert_eq!(row.reel_max_segments, Some(24));
         assert_eq!(row.updated_at, now);
     }
 
@@ -171,6 +186,11 @@ mod tests {
             library: Some("1".to_string()),
             updated_at: now1,
             display_tz: Some("America/New_York".to_string()),
+            default_model: Some("llama3.2:3b".to_string()),
+            default_backend: Some("local".to_string()),
+            default_persona_id: None,
+            reel_span: Some("week".to_string()),
+            reel_max_segments: Some(24),
         };
         dao.upsert_prefs(&ctx(), &prefs1).unwrap();
 
@@ -180,6 +200,11 @@ mod tests {
             library: None,
             updated_at: now2,
             display_tz: None,
+            default_model: None,
+            default_backend: None,
+            default_persona_id: None,
+            reel_span: None,
+            reel_max_segments: None,
         };
         dao.upsert_prefs(&ctx(), &prefs2).unwrap();
 
@@ -188,6 +213,11 @@ mod tests {
         assert_eq!(row.tz_offset_minutes, Some(-300));
         assert!(row.library.is_none());
         assert!(row.display_tz.is_none());
+        // Owner-default columns are overwritten to NULL like the rest.
+        assert!(row.default_model.is_none());
+        assert!(row.default_backend.is_none());
+        assert!(row.reel_span.is_none());
+        assert!(row.reel_max_segments.is_none());
         assert_eq!(row.updated_at, now2);
     }
 
@@ -202,6 +232,11 @@ mod tests {
             library: None,
             updated_at: now,
             display_tz: Some("Europe/Berlin".to_string()),
+            default_model: None,
+            default_backend: None,
+            default_persona_id: Some("default".to_string()),
+            reel_span: Some("month".to_string()),
+            reel_max_segments: None,
         };
         dao.upsert_prefs(&ctx(), &prefs).unwrap();
 
@@ -210,5 +245,9 @@ mod tests {
         assert!(row.voice.is_none());
         assert!(row.library.is_none());
         assert_eq!(row.display_tz, Some("Europe/Berlin".to_string()));
+        assert!(row.default_model.is_none());
+        assert_eq!(row.default_persona_id, Some("default".to_string()));
+        assert_eq!(row.reel_span, Some("month".to_string()));
+        assert!(row.reel_max_segments.is_none());
     }
 }