fix(jlcpcb): skip re-download when cache file already complete (#199, #204)

bhoot found that download_cdfer 416-loops when a complete cdfer.sqlite3 already
exists in the cache (e.g. a prior run downloaded it but died before
conversion/cleanup): resuming a complete file sends Range: bytes=<size>- which
the server answers with HTTP 416, and raise_for_status treated that as a
retryable error, spinning until max_retries then failing.

Fix: HEAD once for Content-Length up front and short-circuit when the local file
already matches the full size; and in the loop, treat a 416 as "complete" when
the size matches (else drop the stale partial and restart fresh). Remove the now
-unused _head_last_modified helper (HEAD is done inline). Add a no-network test.

Verified live: happy path still downloads+converts 616k parts (~50s); a
pre-existing complete file now short-circuits in ~0.4s instead of looping.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_jlcpcb_downloader.py

@@ -266,3 +266,39 @@ def test_result_flags_stale_catalog_and_leaves_fresh_alone(tmp_path, monkeypatch
     assert fresh.get("stale") is not True
     assert "warning" not in fresh
     assert fresh["catalog_age_days"] <= 1
+
+
+def test_download_cdfer_skips_when_file_already_complete(tmp_path, monkeypatch):
+    """A complete cache file must short-circuit, not re-download (no HTTP 416 loop).
+
+    Regression for the force/resume bug: resuming a complete file sends
+    Range: bytes=<size>- which the server answers with 416; the old code retried
+    that until failure. With a HEAD size check we return immediately and never
+    call requests.get.
+    """
+    import requests
+
+    cache_dir = tmp_path / "cache"
+    cache_dir.mkdir()
+    dest = cache_dir / "cdfer.sqlite3"
+    payload = b"x" * 4096
+    dest.write_bytes(payload)
+
+    class _Head:
+        ok = True
+        headers = {
+            "Content-Length": str(len(payload)),
+            "Last-Modified": "Wed, 01 Apr 2026 00:00:00 GMT",
+        }
+
+    monkeypatch.setattr(requests, "head", lambda *a, **k: _Head())
+
+    def _no_get(*a, **k):
+        raise AssertionError("requests.get must not run when the file is already complete")
+
+    monkeypatch.setattr(requests, "get", _no_get)
+
+    path, last_mod = jlcpcb_downloader.download_cdfer(cache_dir)
+    assert path == dest
+    assert path.stat().st_size == len(payload)  # untouched
+    assert last_mod.startswith("Wed, 01 Apr 2026")