Define and apply entity exclusion list for snapshots

### Background

Parent: avniproject/avni-client#1942. Snapshot-server runs against a read replica, so any pulled entity whose sync path issues an `UPDATE` on avni-server breaks the run. Some other entity types must be excluded for separate reasons — per-user/device identity, telemetry (upload-only), or device-local state. Today the snapshot-server omits some of these implicitly; this card makes the full list explicit in code with a single source of truth and a rationale per entry.

### Scope

Add a named constant in snapshot-server listing entity types to exclude. Apply it during snapshot generation. Document the rationale alongside the list.

### Exclusion list

```
IdentifierAssignment - read-replica blocker: pulling this triggers
PrefixedUserPoolBasedIdentifierGenerator,
which UPDATEs identifier_user_assignment to
top up the pool. On a read-only replica that
UPDATE fails and the whole sync request 500s.
Confirmed in prod-read-replica testing.
SyncTelemetry - upload-only (syncPullRequired: false)
VideoTelemetric - upload-only
RuleFailureTelemetry - upload-only
EntityApprovalStatus - syncPullRequired=true in openchs-models, so
this IS pulled by the device on incremental
sync. Safe to exclude from the snapshot
because the first incremental sync after
restore will re-pull it. Verify approval-
gated screens render correctly in that gap.
DraftSubject - device-local
DraftEncounter - device-local
DraftEnrolment - device-local
DraftProgramEncounter - device-local
EntityQueue - device-local pending-upload tracking
MediaQueue - device-local pending-upload tracking
Settings - device-local
LocaleMapping - device-local
MyGroups - per-user; already implicitly excluded
UserSubjectAssignment - per-user; already implicitly excluded
```

`News` is intentionally included. `UserInfo` is intentionally included — required for the client identity check at `BackupRestoreSqliteService.js:110-116`.

### Acceptance criteria

- [ ] Exclusion list is a named constant in snapshot-server code with a one-line rationale per entry.
- [ ] Snapshot generation skips all listed entities (no API fetches for them; no rows in output).
- [ ] Automated assertion: snapshot generated for a fixture user has zero rows for every excluded type.
- [ ] Manual verification (or automated equivalent): a device restored from a snapshot lands on approval-gated screens without errors before the first incremental sync completes; immediately after the first incremental sync, `EntityApprovalStatus` rows are present and screens behave normally.
- [ ] `UserInfo` continues to be included.
- [ ] README note: when avni-client `EntityMetaData` adds a new synced type, evaluate it against this list before next rollout wave.