feat: add global observer IATA whitelist config (#1001)

## Summary

Adds a global `observerIATAWhitelist` config field that restricts which
observer IATA regions are processed by the ingestor.

## Problem

Operators running regional instances (e.g., Sweden) want to ensure only
observers physically in their region contribute data. The existing
per-source `iataFilter` only filters packet messages but still allows
status messages through, meaning observers from other regions appear in
the database.

## Solution

New top-level config field `observerIATAWhitelist`:
- When non-empty, **all** messages (status + packets) from observers
outside the whitelist are silently dropped
- Case-insensitive matching
- Empty list = all regions allowed (fully backwards compatible)
- Lazy O(1) lookup via cached uppercase set (same pattern as
`observerBlacklist`)

### Config example
```json
{
  "observerIATAWhitelist": ["ARN", "GOT"]
}
```

## TDD

- **Red commit:** `f19c2b2` — tests for `ObserverIATAWhitelist` field
and `IsObserverIATAAllowed` method (build fails)
- **Green commit:** `782f516` — implementation + integration test

## Files changed
- `cmd/ingestor/config.go` — new field, new method
`IsObserverIATAAllowed`
- `cmd/ingestor/main.go` — whitelist check in `handleMessage` before
status processing
- `cmd/ingestor/config_test.go` — unit tests for config parsing and
matching
- `cmd/ingestor/main_test.go` — integration test for handleMessage
filtering

Fixes #914

---------

Co-authored-by: you <you@example.com>

config.example.json

@@ -3,6 +3,8 @@
   "apiKey": "your-secret-api-key-here",
   "nodeBlacklist": [],
   "_comment_nodeBlacklist": "Public keys of nodes to hide from all API responses. Use for trolls, offensive names, or nodes reporting false data that operators refuse to fix.",
+  "observerIATAWhitelist": [],
+  "_comment_observerIATAWhitelist": "Global IATA region whitelist. When non-empty, only observers whose IATA code (from MQTT topic) matches are processed. Case-insensitive. Empty = allow all. Unlike per-source iataFilter, this applies across all MQTT sources.",
   "retention": {
     "nodeDays": 7,
     "observerDays": 14,