fix: show true lifetime totals in the Sessions widget

[ntatschner]

Problem

The Sessions widget's summary line ("N sessions · Xh played", labelled lifetime) was computed from the session list endpoint, which caps at SESSIONS_LIST_LIMIT = 50 newest-first sessions. So the count pinned at "50" and the hours only summed the 50 newest sessions — a large undercount for anyone who's played more (reported live on v1.8.52).

Fix

Backend (query.rs): add an all_time flag to GET /v1/me/stats/playtime (new PlaytimeParams). When set, it aggregates over all history (since = None) instead of the bounded hours window (which is capped at 1 year and can't represent true lifetime). hours = 0 in the response is the all-time sentinel. The underlying total_playtime_secs(None) / count_sessions_since(None) already supported this — only the HTTP surface was missing it.

Frontend: the profile owner now fetches the all-time aggregate and the widget shows true lifetime session_count + playtime. Visitors (no me-scoped lifetime endpoint exists) fall back to the capped list, labelled N+ so the number doesn't read as exact. Summary logic extracted to a pure buildSessionSummary() with unit tests.

Scope note

Owner-first by design (per discussion). Accurate lifetime totals for visitors viewing someone else's profile would need a new handle-scoped /v1/users/{handle}/stats/playtime (sharing-auth) — deliberately deferred.

Test plan

• cargo test -p starstats-server — new stats_playtime_all_time_includes_sessions_outside_window (counts a 60-day-old session the default window excludes) + existing playtime test pass
• cargo clippy -p starstats-server clean; cargo fmt --check clean
• pnpm --filter web test:run — 248 pass incl. 4 new buildSessionSummary cases (lifetime, 50+ cap label, exact count, singular)
• pnpm --filter web typecheck + lint clean
• OpenAPI client regenerated (all_time query param added)
• e2e: sessions widget specs are test.skip'd (NEXT_PUBLIC_PROFILE_WIDGETS not set in playwright.config) — no drift

⚠️ Roadmap label omitted: roadmap/game-running-detection-session-metrics-anti-cheat-safe exceeds GitHub's 50-char label limit. At release time pass --roadmap-item-slug game-running-detection-session-metrics-anti-cheat-safe manually.

Roadmap-Item: game-running-detection-session-metrics-anti-cheat-safe