Support idempotent offline queue recovery imports

[stef-k]

Summary

• accept the mobile queue idempotency GUID in CSV and Wayfarer GeoJSON location imports
• converge bulk imports and normal authenticated location delivery on the same per-user record
• preserve legacy keyless import behavior and existing PostgreSQL uniqueness ownership
• document the recovery/import identity and ownership boundaries

Coordinates with stef-k/WayfarerMobile#244.

Validation

• focused parser/import/API/convergence selection: 51 passed
• guarded PostgreSQL concurrency: 1 passed; zero fixture residue
• Debug build: succeeded with 0 warnings and 0 errors
• LOC policy: passed; only pre-existing unrelated warnings
• git diff --check: passed

Database

No migration. This reuses the existing nullable idempotency-key column and per-user unique index.