Add connectivity check to diagnose why all metadata APIs return 0 hits

All four sources returning 0 results is a strong signal of a network
problem (DNS, firewall, proxy, Docker isolation) rather than four
independent matching bugs.

New endpoint GET /api/items/match/connectivity:
- Pings Google, MusicBrainz, OpenLibrary, GoogleBooks, DNB
- Returns per-target HTTP status, byte count, latency, error
- Surfaces any HTTP_PROXY / HTTPS_PROXY env vars that httpx would use

UI: New "Connectivity-Check" button in BookDetail. Result panel shows
each target green (HTTP 200) or red (error/timeout), so the user can
immediately see whether the backend has outbound internet access at
all, or whether it's a per-API issue.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/matching.py

@@ -52,6 +52,60 @@ async def search_match(
     return {"results": results}
 
 
+@router.get("/match/connectivity")
+async def check_connectivity(
+    current_user: User = Depends(get_current_user),
+):
+    """Testet ob das Backend die externen Metadaten-APIs erreichen kann."""
+    import httpx
+    import time
+
+    targets = [
+        ("Google", "https://www.google.com"),
+        ("MusicBrainz", "https://musicbrainz.org/ws/2/release?query=test&fmt=json&limit=1"),
+        ("OpenLibrary", "https://openlibrary.org/search.json?title=test&limit=1"),
+        ("GoogleBooks", "https://www.googleapis.com/books/v1/volumes?q=test&maxResults=1"),
+        ("DNB", "https://services.dnb.de/sru/dnb?version=1.1&operation=searchRetrieve&query=tit%3Dtest&maximumRecords=1"),
+    ]
+    headers = {"User-Agent": "audiolib/1.0"}
+
+    results = []
+    async with httpx.AsyncClient(headers=headers, timeout=15, follow_redirects=True) as client:
+        for name, url in targets:
+            t0 = time.time()
+            try:
+                r = await client.get(url)
+                results.append({
+                    "name": name,
+                    "url": url,
+                    "ok": True,
+                    "status": r.status_code,
+                    "bytes": len(r.content),
+                    "ms": int((time.time() - t0) * 1000),
+                    "body_snippet": (r.text[:150] if r.status_code != 200 else None),
+                })
+            except Exception as e:
+                results.append({
+                    "name": name,
+                    "url": url,
+                    "ok": False,
+                    "error": f"{type(e).__name__}: {e}",
+                    "ms": int((time.time() - t0) * 1000),
+                })
+
+    # Auch Env-Variablen die httpx beeinflussen
+    import os
+    proxy_env = {
+        k: v for k, v in os.environ.items()
+        if k.upper() in ("HTTP_PROXY", "HTTPS_PROXY", "NO_PROXY", "ALL_PROXY")
+    }
+
+    return {
+        "results": results,
+        "proxy_env": proxy_env or "keine",
+    }
+
+
 @router.get("/match/debug")
 async def debug_match(
     title: str,