Phase 5: ESI Expires caching, error-limit back-off, setup docs (#16)

EsiClient now caches GET responses honoring the Expires header and backs off
when the rolling ESI error window is nearly exhausted (X-Esi-Error-Limit-*).
README gains Setup / Connect-to-Claude / enabling-writes sections.

Closes #16. SDE bulk-load split out to its own follow-up (#51).
135 tests, coverage 90%.

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

tests/test_esi.py

@@ -8,6 +8,54 @@ import pytest
 from familiar import esi
 
 
+_FUTURE = "Wed, 21 Oct 2099 07:28:00 GMT"
+
+
+def test_get_caches_when_expires_present(mock_esi):
+    calls = {"n": 0}
+
+    def handler(req):
+        calls["n"] += 1
+        return httpx.Response(200, json={"v": 1}, headers={"Expires": _FUTURE})
+
+    mock_esi(handler)
+    first = asyncio.run(esi.client().get("/x/"))
+    second = asyncio.run(esi.client().get("/x/"))
+    assert first == second == {"v": 1}
+    assert calls["n"] == 1  # second served from cache
+
+
+def test_no_cache_without_expires(mock_esi):
+    calls = {"n": 0}
+
+    def handler(req):
+        calls["n"] += 1
+        return httpx.Response(200, json={"v": 1})
+
+    mock_esi(handler)
+    asyncio.run(esi.client().get("/y/"))
+    asyncio.run(esi.client().get("/y/"))
+    assert calls["n"] == 2
+
+
+def test_error_limit_triggers_backoff(mock_esi, monkeypatch):
+    slept = []
+
+    async def fake_sleep(s):
+        slept.append(s)
+
+    monkeypatch.setattr(esi, "_sleep", fake_sleep)
+    mock_esi(
+        lambda req: httpx.Response(
+            200, json={}, headers={"X-Esi-Error-Limit-Remain": "1", "X-Esi-Error-Limit-Reset": "3"}
+        )
+    )
+    c = esi.client()
+    asyncio.run(c.get("/a/"))  # records remain=1
+    asyncio.run(c.get("/b/"))  # next request backs off before firing
+    assert slept and slept[0] > 0
+
+
 def test_get_raises_esi_error_on_http_error(mock_esi):
     mock_esi(lambda req: httpx.Response(404, json={"error": "not found"}))
     with pytest.raises(esi.EsiError) as exc: