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.

CLAUDE.md

@@ -533,6 +533,11 @@ POST /insights/chat                  (single-turn reply, non-streaming)
 POST /insights/chat/stream           (SSE: text / tool_call / tool_result / truncated / done)
 GET  /insights/chat/history?path=... (rendered transcript with tool invocations)
 POST /insights/chat/rewind           (truncate transcript at a rendered index)
+
+// User AI Prefs (owner defaults + passive mirror; src/prefs.rs)
+GET  /prefs                          (current row; auth)
+PUT  /prefs                          (partial update: omitted=unchanged, null=clear, value=set; auth)
+GET  /timezones                      (IANA zone list from chrono-tz; unauthenticated)
 ```
 
 **Request Types:**