faces: ignore/junk bucket — DB schema + lazy-create endpoint

A single global "Ignored" person row, marked is_ignored=true, that the
frontend lazily creates on first use to hold strangers, false
detections, and faces the user doesn't want bound to a real person.

Schema (new migration 2026-04-29-000200_add_is_ignored):
  - persons.is_ignored BOOLEAN NOT NULL DEFAULT 0
  - Partial index on (is_ignored) WHERE is_ignored = 1; small WHERE
    set means a tiny index that only ever services the bucket lookup.

Why a real persons row instead of a separate table or status enum:
  - face_detections.person_id stays a clean foreign key — no special
    code paths for "ignored faces" anywhere else in the schema.
  - The cluster-suggester already filters by `person_id IS NULL`, so
    bound-to-ignored faces are naturally excluded from re-clustering
    without any change.
  - merge / rename / delete all work on it with the existing routes
    (the management UI just hides it from default views).

DAO additions / changes:
  - get_or_create_ignored_person (idempotent; race-safe via the
    UNIQUE COLLATE NOCASE on persons.name + retry-on-409 fallback).
  - list_persons gains an include_ignored parameter; default false
    so the management screen hides the bucket unless asked.
  - find_persons_by_names_ci filters is_ignored=0 in SQL so the
    auto-bind path can NEVER target the bucket — even if the user
    happens to tag photos as "Ignored", the heuristic look-up skips
    it. Bucket assignment is always an explicit operator action.
  - update_person accepts is_ignored: Option<bool> so a person can
    be moved into / out of the bucket without a delete + recreate.

Routes:
  - POST /persons/ignore-bucket — returns the bucket, creating it on
    first call. Frontend uses this lazily right before binding.
  - GET /persons gains ?include_ignored=true; default behavior
    unchanged.
  - PATCH /persons/{id} now accepts is_ignored.

Tests: ignore_bucket_idempotent_and_filters_auto_bind covers the
contract: bucket is idempotent across calls, find_persons_by_names_ci
skips it (even on exact name match), default list_persons hides it,
include_ignored=true surfaces it. All other tests updated to pass
the new is_ignored: false / Option<bool> fields explicitly.

cargo test --lib: 181/0; fmt + clippy clean for new code.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrations/2026-04-29-000200_add_is_ignored/down.sql

@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS idx_persons_is_ignored;
+ALTER TABLE persons DROP COLUMN is_ignored;