tests: add unit test infrastructure

pytest-based test suite with fixtures for database testing.
Covers misc.py utilities, dbs.py operations, and fetch.py validation.
Includes mock_network.py for future network testing.

tests/fixtures/sample_mixed.txt

@@ -0,0 +1,24 @@
+Free Proxy List - Updated Daily
+
+Plain proxies:
+1.2.3.4:8080
+5.6.7.8:3128
+9.10.11.12:1080
+
+With protocol hints:
+socks5 11.22.33.44:1080
+http: 55.66.77.88:8080
+socks4 - 99.100.101.102:1080
+
+Auth proxies:
+user:pass@111.112.113.114:8080
+socks5://admin:secret@115.116.117.118:1080
+
+JSON embedded:
+{"data": [{"ip": "121.122.123.124", "port": 8080}]}
+
+Table format:
+<table>
+<tr><th>IP</th><th>Port</th></tr>
+<tr><td>131.132.133.134</td><td>3128</td></tr>
+</table>