Adds snapshot provider that queries the latest non-faulty position per device
registered to an event, returned in the `subscribed` reply so the SPA map is
populated immediately rather than waiting for the first live broadcast batch.
Key changes:
- src/live/snapshot.ts: createSnapshotProvider factory using DISTINCT ON
(device_id) ... ORDER BY device_id, ts DESC with WHERE faulty=false; converts
Date ts to epoch ms; omits speed/course when 0 (matching broadcast convention)
- src/main.ts: injects createSnapshotProvider(pool) into createSubscriptionRegistry
- test/live-snapshot.test.ts: 7 unit tests covering: two-device result, empty
event, faulty exclusion, DISTINCT ON semantics, parameterized query, metrics
observation, and error propagation
The snapshot query requires the positions_device_ts_idx created in migration 0002
(task 1.5.4). Snapshot failures fail open — registry.fetchSnapshot returns [] so
the subscription still succeeds with an empty initial state.
