docs(attribution): credit adsbdb and carry its route-data terms

adsbdb supplies the aircraft type, model name and registration behind every
enriched flight, and the airline and origin/destination pair behind the
tracked contact's FROM -> TO strip. It appeared in neither attribution
surface: DATA_SOURCES.md and src/data/dataCredits.js each held zero
occurrences of it, against the rule both files state for themselves and
CONTRIBUTING's fourth pull-request item. adsb.lol is a different operator and
its ODbL credit does not reach this data.

The reporter of #256 left the patch off deliberately, saying the terms were
what should not be guessed. They are published, so this takes them from the
source rather than inventing them: adsbdb credits PlaneBase for the aircraft
data and Guillaume Michel for the ICAO-to-N-number conversion, and states that
the flight route data "is the work of David Taylor, Edinburgh and Jim Mason,
Glasgow, and may not be copied, published, or incorporated into other
databases without the explicit permission of David J Taylor, Edinburgh". No
licence or rate limit is published for the service itself.

That restriction is the reason the note records what the proxy does with what
it fetches: responses are cached per key for 24 hours in .gev-cache/adsbdb.json,
which is gitignored, so the data stays on the operator's machine and is never
bundled with this repo or redistributed.

A test pins the credit. The attribution is a requirement rather than a
preference, so it should fail loudly if a later edit drops the names or the
restriction, instead of surviving on luck.

src/data/dataCredits.js

@@ -42,6 +42,16 @@ export const DATA_CREDITS = [
       '<a href="https://adsb.lol" target="_blank" rel="noopener">adsb.lol</a> ' +
       '(ODbL 1.0)',
   },
+  {
+    key: 'adsbdb',
+    html:
+      'Aircraft type, registration &amp; flight routes: ' +
+      '<a href="https://www.adsbdb.com" target="_blank" rel="noopener">adsbdb</a> ' +
+      '· aircraft data from PlaneBase · route data is the work of ' +
+      'David Taylor, Edinburgh, and Jim Mason, Glasgow, and may not be ' +
+      'copied, published, or incorporated into other databases without the ' +
+      'explicit permission of David J Taylor, Edinburgh',
+  },
   {
     key: 'aisstream',
     html: