chore(t-paliad-081): doc + dead-code batch (F-5/F-10/F-11/F-15/F-16/F-17/F-18)

Bundle of small audit findings, all doc-only or dead-code:

- F-5: refresh stale escalation-contact comment in models.User —
  Settings UI dropdown shipped 2026-04-29 (t-paliad-066).
- F-10: add "OBSOLETED by migration 018" note to migrations 004/005/006
  so readers stop hunting for the live shape in obsolete files.
- F-11: document the data-loss semantics of dropping
  paliad.partner_unit_events on the 027 down — audit rows are
  append-only telemetry, accepted loss on rollback.
- F-15: drop the patholo_session / patholo_refresh cookie fallback
  added during the 2026-04-16 rebrand. Active users have long since
  been re-authed through the upgrade path; inactive users hit the
  normal /login flow.
- F-16: refresh stale /api/departments comment in team_pages.go to
  /api/partner-units (renamed in t-paliad-070).
- F-17: move internal/db/migrations/_dev/mock_supabase_auth.sql to
  internal/db/devtools/ so a future loosening of the //go:embed
  pattern can't accidentally ship the dev-only fixture.
- F-18: update docs/project-status.md "Audit polish-2" entry — the
  batch shipped via t-paliad-067 / 068 / 073, follow-ups are now
  tracked under the 2026-04-30 re-audit + t-paliad-074.

go build / vet / test clean.

docs/project-status.md

@@ -17,7 +17,7 @@ Phases A–G shipped (April 2026): schema + RLS, services, Fristenrechner→DB,
 ## Open follow-ups
 
 - **Settings → Notifications: escalation contact dropdown** — migration 025 ships `paliad.users.escalation_contact_id` (FK to `paliad.users`, nullable, ON DELETE SET NULL). NULL means "fall back to global_admins for the escalation channel"; setting it lets a user designate a specific colleague as their escalation contact. UI shipped t-paliad-066 on 2026-04-29.
-- **Audit polish-2** — ~25 BATCH-level findings from `docs/audit-polish-2026-04-27.md` not yet shipped (PR-A..PR-E covered the rest). Triage in flight as t-paliad-067.
+- **Audit polish-2** — shipped 2026-04-30 across t-paliad-067 / t-paliad-068 / t-paliad-073 (BATCH-level findings + DEFER list). Follow-ups from the 2026-04-30 re-audit (`docs/improvement-audit-2026-04-30.md`) are tracked under t-paliad-074 and downstream task IDs.
 - **KanzlAI infra retirement** — Dokploy shutdown, `kanzlai` schema drop, Gitea archive. Pending m + head coordination.
 
 ## Historical naming