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/models.rs

@@ -579,6 +579,19 @@ pub struct UserAiPrefs {
     /// default (`camera_tz::DEFAULT_DISPLAY_TZ`). The legacy
     /// `tz_offset_minutes` column stays frozen for rollback.
     pub display_tz: Option<String>,
+    /// Last/explicit insight + chat model choice (local id or OpenRouter id).
+    /// NULL = current per-request / env default behavior.
+    pub default_model: Option<String>,
+    /// `"local"` | `"hybrid"` — insight/chat backend. NULL = per-request /
+    /// hard default ("local").
+    pub default_backend: Option<String>,
+    /// Active persona for persona chat. NULL = hard default ("default").
+    pub default_persona_id: Option<String>,
+    /// Reel span: `"day"` | `"week"` | `"month"`. NULL = per-request /
+    /// hard default (day).
+    pub reel_span: Option<String>,
+    /// Reel segment cap. NULL = per-request / hard default.
+    pub reel_max_segments: Option<i32>,
 }
 
 #[derive(Insertable, Debug, Clone, serde::Deserialize, serde::Serialize)]
@@ -589,4 +602,9 @@ pub struct UpsertUserAiPrefs {
     pub library: Option<String>,
     pub updated_at: i64,
     pub display_tz: Option<String>,
+    pub default_model: Option<String>,
+    pub default_backend: Option<String>,
+    pub default_persona_id: Option<String>,
+    pub reel_span: Option<String>,
+    pub reel_max_segments: Option<i32>,
 }