test: cover routers, auth, rate limit, config_manager, imgur extractor

- tests/test_auth.py: enable/disable via env, valid/invalid creds
- tests/test_rate_limit.py: bucket math + window expiry
- tests/test_router_media.py: path traversal (file/thumb), paginated subreddits/authors, blacklist preview shape, DELETE flow, cleanup-by-type 400 paths
- tests/test_router_stats.py: cache TTL, .json exclusion, missing dir
- tests/test_router_favorites.py: paginated favorites/authors, no N+1 (query count), favorites_only filter
- tests/test_router_config.py: CRUD subreddits/users/blacklist
- tests/test_config_manager.py: case-insensitive dedupe, blacklist removes from collection, domain normalization
- tests/test_extractors_imgur.py: i.imgur direct, gifv->mp4, albums/galleries -> None

src/database.py: resolve RMC_DB_PATH lazily in Database() so tests can monkeypatch the env per case (no more module-import capture)

Result: 135 passed (was 77). Coverage 55% global; 100% on auth/imgur, 96% rate_limit, 84% stats router, 80% config_manager.

tests/test_extractors_imgur.py

@@ -0,0 +1,42 @@
+"""Tests for the Imgur URL extractor."""
+
+from __future__ import annotations
+
+import pytest
+
+from src.extractors.imgur import extract_imgur_url
+
+
+@pytest.mark.parametrize(
+    "url,expected_url,expected_type",
+    [
+        ("https://i.imgur.com/abc123.jpg", "https://i.imgur.com/abc123.jpg", "image"),
+        ("https://i.imgur.com/abc123.png", "https://i.imgur.com/abc123.png", "image"),
+        (
+            "https://i.imgur.com/abc123.gifv",
+            "https://i.imgur.com/abc123.mp4",
+            "video",
+        ),
+        ("https://imgur.com/abc123", "https://i.imgur.com/abc123.jpg", "image"),
+        ("https://imgur.com/abc123.jpg", "https://i.imgur.com/abc123.jpg", "image"),
+    ],
+)
+def test_extract_imgur_url_variants(url, expected_url, expected_type):
+    result, media_type = extract_imgur_url(url)
+    assert result == expected_url
+    assert media_type == expected_type
+
+
+def test_album_returns_none():
+    result, _ = extract_imgur_url("https://imgur.com/a/abc123")
+    assert result is None
+
+
+def test_gallery_returns_none():
+    result, _ = extract_imgur_url("https://imgur.com/gallery/abc123")
+    assert result is None
+
+
+def test_path_with_no_id_returns_none():
+    result, _ = extract_imgur_url("https://imgur.com/")
+    assert result is None