feat: add SQLite stats database layer for CivitAI stats

Implements StatsDB class with upsert, batch upsert, get_by_hashes,
get_all, count, and close methods backed by a WAL-mode SQLite database.
Also guards __init__.py relative imports behind __package__ check so
pytest can run without ComfyUI context, and removes empty tests/__init__.py
to prevent pytest Package collector from traversing into the plugin root.

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

tests/test_stats_db.py

@@ -0,0 +1,99 @@
+"""Tests for the CivitAI stats database layer."""
+import pytest
+from pathlib import Path
+
+
+@pytest.fixture
+def db_path(tmp_path):
+    return tmp_path / "test_stats.db"
+
+
+@pytest.fixture
+def stats_db(db_path):
+    from stats_db import StatsDB
+    db = StatsDB(db_path)
+    db.init()
+    return db
+
+
+def test_init_creates_table(stats_db, db_path):
+    """DB file is created and table exists."""
+    assert db_path.exists()
+
+
+def test_upsert_and_get_single(stats_db):
+    """Insert a stat row and retrieve it by sha256."""
+    stats_db.upsert("abc123", {
+        "civitai_model_id": 1,
+        "civitai_version_id": 10,
+        "download_count": 5000,
+        "rating": 4.8,
+        "rating_count": 120,
+        "thumbs_up_count": 89,
+    })
+    result = stats_db.get_by_hashes(["abc123"])
+    assert "abc123" in result
+    assert result["abc123"]["download_count"] == 5000
+    assert result["abc123"]["rating"] == 4.8
+    assert result["abc123"]["thumbs_up_count"] == 89
+
+
+def test_upsert_updates_existing(stats_db):
+    """Upserting same sha256 updates values."""
+    stats_db.upsert("abc123", {"download_count": 100, "rating": 3.0,
+                                "rating_count": 10, "thumbs_up_count": 5})
+    stats_db.upsert("abc123", {"download_count": 200, "rating": 4.0,
+                                "rating_count": 20, "thumbs_up_count": 15})
+    result = stats_db.get_by_hashes(["abc123"])
+    assert result["abc123"]["download_count"] == 200
+
+
+def test_get_by_hashes_batch(stats_db):
+    """Batch retrieval returns only matching hashes."""
+    for i in range(5):
+        stats_db.upsert(f"hash_{i}", {"download_count": i * 100,
+                                       "rating": 0, "rating_count": 0,
+                                       "thumbs_up_count": 0})
+    result = stats_db.get_by_hashes(["hash_1", "hash_3", "nonexistent"])
+    assert len(result) == 2
+    assert "hash_1" in result
+    assert "hash_3" in result
+    assert "nonexistent" not in result
+
+
+def test_get_by_hashes_empty_input(stats_db):
+    """Empty input returns empty dict."""
+    assert stats_db.get_by_hashes([]) == {}
+
+
+def test_upsert_batch(stats_db):
+    """Batch upsert inserts multiple rows."""
+    rows = [
+        ("h1", {"civitai_model_id": 1, "download_count": 100,
+                "rating": 4.0, "rating_count": 10, "thumbs_up_count": 5}),
+        ("h2", {"civitai_model_id": 2, "download_count": 200,
+                "rating": 3.5, "rating_count": 20, "thumbs_up_count": 15}),
+    ]
+    stats_db.upsert_batch(rows)
+    result = stats_db.get_by_hashes(["h1", "h2"])
+    assert len(result) == 2
+    assert result["h1"]["download_count"] == 100
+    assert result["h2"]["download_count"] == 200
+
+
+def test_get_all_stats(stats_db):
+    """get_all returns every row keyed by sha256."""
+    stats_db.upsert("a", {"download_count": 1, "rating": 0,
+                           "rating_count": 0, "thumbs_up_count": 0})
+    stats_db.upsert("b", {"download_count": 2, "rating": 0,
+                           "rating_count": 0, "thumbs_up_count": 0})
+    result = stats_db.get_all()
+    assert len(result) == 2
+
+
+def test_stats_count(stats_db):
+    """count returns number of rows."""
+    assert stats_db.count() == 0
+    stats_db.upsert("a", {"download_count": 1, "rating": 0,
+                           "rating_count": 0, "thumbs_up_count": 0})
+    assert stats_db.count() == 1