feat: replace per-author cap with interleaving to avoid dropping posts in feed and home

The old `diversify_by_author` capped each author to 2 posts per page, silently dropping excess rows. The new `interleave_by_author` reorders the full result set so no two consecutive posts share an author, preserving every post while breaking up same-author runs. `paginate_diverse` loses its `max_per_author` and `pool` parameters; the home route now fetches exactly 6 rows and interleaves them instead of over-fetching 50 and capping. Tests are rewritten to verify interleaving behavior and per-author chronological order preservation.

tests/unit/database.py

@@ -185,29 +185,40 @@ def test_build_pagination_metadata():
     assert clamped["total_pages"] == 1
 
 
-def test_diversify_by_author_caps_per_author():
-    from devplacepy.database import diversify_by_author
+def test_interleave_by_author_spreads_consecutive_runs():
+    from devplacepy.database import interleave_by_author
 
     rows = [
         {"uid": 1, "user_uid": "a"},
         {"uid": 2, "user_uid": "a"},
         {"uid": 3, "user_uid": "a"},
         {"uid": 4, "user_uid": "b"},
-        {"uid": 5, "user_uid": "a"},
+        {"uid": 5, "user_uid": "b"},
         {"uid": 6, "user_uid": "c"},
     ]
-    kept = diversify_by_author(rows, max_per_author=2, limit=6)
-    authors = [r["user_uid"] for r in kept]
-    assert authors.count("a") == 2
-    assert authors.count("b") == 1
-    assert authors.count("c") == 1
-    assert [r["uid"] for r in kept] == [1, 2, 4, 6]
+    spread = interleave_by_author(rows)
+    authors = [r["user_uid"] for r in spread]
+    assert len(spread) == len(rows)
+    assert all(authors[i] != authors[i + 1] for i in range(len(authors) - 1))
 
 
-def test_diversify_by_author_respects_limit():
-    from devplacepy.database import diversify_by_author
+def test_interleave_by_author_preserves_per_author_order():
+    from devplacepy.database import interleave_by_author
 
-    rows = [{"uid": i, "user_uid": str(i)} for i in range(10)]
-    kept = diversify_by_author(rows, max_per_author=2, limit=3)
-    assert len(kept) == 3
-    assert [r["uid"] for r in kept] == [0, 1, 2]
+    rows = [
+        {"uid": 1, "user_uid": "a"},
+        {"uid": 2, "user_uid": "a"},
+        {"uid": 3, "user_uid": "a"},
+        {"uid": 4, "user_uid": "b"},
+    ]
+    spread = interleave_by_author(rows)
+    a_order = [r["uid"] for r in spread if r["user_uid"] == "a"]
+    assert a_order == [1, 2, 3]
+
+
+def test_interleave_by_author_single_author_keeps_order():
+    from devplacepy.database import interleave_by_author
+
+    rows = [{"uid": i, "user_uid": "a"} for i in range(4)]
+    spread = interleave_by_author(rows)
+    assert [r["uid"] for r in spread] == [0, 1, 2, 3]