Add User-Agent header to Wikidata API requests (#693)

Fixes #692

This will send a user agent header of the form
```
OpenRailwayMap API (https://openrailwaymap.app), httpx 0.28.1, Python 3.13.9 (main, Oct 15 2025, 16:47:14) [GCC 14.2.0]
```

Now it works
```shell
http http://localhost:8000/api/wikidata/Q567336

HTTP/1.1 307 Temporary Redirect
Cache-Control: public, max-age=0, stale-if-error=0
Connection: keep-alive
Content-Length: 0
Date: Sun, 30 Nov 2025 14:46:38 GMT
Server: nginx/1.29.3
location: https://upload.wikimedia.org/wikipedia/commons/thumb/8/81/Bahnhof_Berlin-Wannsee_Empfangsgebaeude_04-2015.jpg/330px-Bahnhof_Berlin-Wannsee_Empfangsgebaeude_04-2015.jpg
```

Popup also shows images again
<img width="657" height="543" alt="image"
src="https://github.com/user-attachments/assets/79ccb99d-8de1-4591-9aba-27bda2cfa23a"
/>

Author: hiddewie

api/api.py

@@ -1,6 +1,7 @@
 import contextlib
 import os
 from typing import Annotated
+import sys
 
 import asyncpg
 from fastapi import FastAPI
@@ -14,6 +15,9 @@
 from openrailwaymap_api.replication_api import ReplicationAPI
 from openrailwaymap_api.wikidata_api import WikidataAPI
 
+DEFAULT_HTTP_HEADERS = {
+  'User-Agent': f'OpenRailwayMap API (https://openrailwaymap.app), httpx {httpx.__version__}, Python {sys.version}'
+}
 
 @contextlib.asynccontextmanager
 async def lifespan(app):
@@ -28,7 +32,7 @@ async def lifespan(app):
         print('Connected to database')
         app.state.database = pool
 
-        async with httpx.AsyncClient(timeout=3.0) as http_client:
+        async with httpx.AsyncClient(timeout=3.0, headers=DEFAULT_HTTP_HEADERS) as http_client:
             print('Created HTTP client')
             app.state.http_client = http_client