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.

migrations/2026-09-27-000000_add_owner_defaults/down.sql

@@ -0,0 +1,5 @@
+ALTER TABLE user_ai_prefs DROP COLUMN default_model;
+ALTER TABLE user_ai_prefs DROP COLUMN default_backend;
+ALTER TABLE user_ai_prefs DROP COLUMN default_persona_id;
+ALTER TABLE user_ai_prefs DROP COLUMN reel_span;
+ALTER TABLE user_ai_prefs DROP COLUMN reel_max_segments;