feat(memories): Tier 2 — people_together memory type (pair co-occurrence)

[Deeds67]

What

Adds the first Tier 2 memory type — people_together: a pair of people (or pets) who were photographed together a lot in the current calendar month of a past year. Titled "Anna & Ben", subtitled "18 photos together · June 2023".

Reframed from the roadmap's "You & [person]" (#5) to a pair memory — the schema has no reliable account-owner→person mapping, so "You & X" would need a fragile heuristic. Date-anchored like month_recap: fires on day 20 with a 7-day visibility window.

Spec: docs/plans/2026-07-16-memory-types-tier2-people-together-spec.md

How — 4 TDD slices

1. pairCounts curation helper — pure pair co-occurrence (dedupes a person's multiple faces per asset, no self-pairs), distinct-day counting, deterministic ordering.
2. getMemoryFacesForPeriod repository query — flat face rows for named, non-hidden people and pets; @GenerateSql snapshot regenerated.
3. people_together rule (end-to-end) — rule + registry + metadata + admin toggle + 4 en.json keys.
4. End-to-end generation medium test (positive + negative) + roadmap status.

Key decisions

• Pair, not "You & X" — no owner→person heuristic needed.
• Fires day 20 / 7-day window — staggered after month_recap (day 1) and favorites_throwback (day 15) so it never starves month_recap's single day-1 slot. Zero engine change — reuses the visibleForDays machinery shipped in Tier 1.
• Thresholds: ≥ 6 co-occurring photos across ≥ 2 distinct days (a single event doesn't qualify a "relationship").
• Pets included — Anna & Rex and Rex & Whiskers both possible.
• Title ordered by person id — deterministic; two people can share a name.

Tests

• Unit: pairCounts (9 cases), rule spec (13 cases, exact score pinned), metadata/registry — all green.
• Medium (real DB): 5 getMemoryFacesForPeriod filter tests + 2 end-to-end generation tests (positive/negative, non-tautology verified).
• tsc / eslint --max-warnings 0 / prettier clean; SQL snapshot regenerated (clean diff); mobile parity N/A (renders rule memories generically — verified).

Stacked on #789

Based on feat/memory-types-tier1 (Tier 1, PR #789). GitHub will auto-retarget this to main once #789 merges.