ppf: add content hash for duplicate proxy list detection

dbs.py

@@ -37,6 +37,31 @@ def _migrate_asn_column(sqlite):
         sqlite.commit()
 
 
+def _migrate_content_hash_column(sqlite):
+    """Add content_hash column to uris table for duplicate detection."""
+    try:
+        sqlite.execute('SELECT content_hash FROM uris LIMIT 1')
+    except Exception:
+        sqlite.execute('ALTER TABLE uris ADD COLUMN content_hash TEXT')
+        sqlite.commit()
+
+
+def compute_proxy_list_hash(proxies):
+    """Compute MD5 hash of sorted proxy list for change detection.
+
+    Args:
+        proxies: List of proxy strings (ip:port format)
+
+    Returns:
+        Hexadecimal MD5 hash string, or None if list is empty
+    """
+    if not proxies:
+        return None
+    import hashlib
+    sorted_list = '\n'.join(sorted(proxies))
+    return hashlib.md5(sorted_list.encode('utf-8') if hasattr(sorted_list, 'encode') else sorted_list).hexdigest()
+
+
 def update_proxy_latency(sqlite, proxy, latency_ms):
     """Update rolling average latency for a proxy.
 
@@ -159,7 +184,10 @@ def create_table_if_not_exists(sqlite, dbname):
             stale_count INT,
             retrievals INT,
             proxies_added INT,
-            added INT)""")
+            added INT,
+            content_hash TEXT)""")
+        # Migration for existing databases
+        _migrate_content_hash_column(sqlite)
         # Indexes for common query patterns
         sqlite.execute('CREATE INDEX IF NOT EXISTS idx_uris_error ON uris(error)')
         sqlite.execute('CREATE INDEX IF NOT EXISTS idx_uris_checktime ON uris(check_time)')