add latency tracking and dynamic thread scaling

- dbs.py: add avg_latency, latency_samples columns with migration
- dbs.py: update_proxy_latency() with exponential moving average
- proxywatchd.py: ThreadScaler class for dynamic thread count
- proxywatchd.py: calculate/record latency for successful proxies
- proxywatchd.py: _spawn_thread(), _remove_thread(), _adjust_threads()
- scaler reports status alongside periodic stats

dbs.py

@@ -6,6 +6,46 @@ import time
 from misc import _log
 
 
+def _migrate_latency_columns(sqlite):
+    """Add latency columns to existing databases."""
+    try:
+        sqlite.execute('SELECT avg_latency FROM proxylist LIMIT 1')
+    except Exception:
+        sqlite.execute('ALTER TABLE proxylist ADD COLUMN avg_latency REAL DEFAULT 0')
+        sqlite.execute('ALTER TABLE proxylist ADD COLUMN latency_samples INT DEFAULT 0')
+        sqlite.commit()
+
+
+def update_proxy_latency(sqlite, proxy, latency_ms):
+    """Update rolling average latency for a proxy.
+
+    Args:
+        sqlite: Database connection
+        proxy: Proxy address (ip:port)
+        latency_ms: Response latency in milliseconds
+    """
+    row = sqlite.execute(
+        'SELECT avg_latency, latency_samples FROM proxylist WHERE proxy=?',
+        (proxy,)
+    ).fetchone()
+
+    if row:
+        old_avg, samples = row[0] or 0, row[1] or 0
+        # Exponential moving average, capped at 100 samples
+        new_samples = min(samples + 1, 100)
+        if samples == 0:
+            new_avg = latency_ms
+        else:
+            # Weight recent samples more heavily
+            alpha = 2.0 / (new_samples + 1)
+            new_avg = alpha * latency_ms + (1 - alpha) * old_avg
+
+        sqlite.execute(
+            'UPDATE proxylist SET avg_latency=?, latency_samples=? WHERE proxy=?',
+            (new_avg, new_samples, proxy)
+        )
+
+
 def create_table_if_not_exists(sqlite, dbname):
     """Create database table with indexes if it doesn't exist."""
     if dbname == 'proxylist':
@@ -22,11 +62,15 @@ def create_table_if_not_exists(sqlite, dbname):
             ip TEXT,
             port INT,
             consecutive_success INT,
-            total_duration INT)""")
+            total_duration INT,
+            avg_latency REAL DEFAULT 0,
+            latency_samples INT DEFAULT 0)""")
         # Indexes for common query patterns
         sqlite.execute('CREATE INDEX IF NOT EXISTS idx_proxylist_failed ON proxylist(failed)')
         sqlite.execute('CREATE INDEX IF NOT EXISTS idx_proxylist_tested ON proxylist(tested)')
         sqlite.execute('CREATE INDEX IF NOT EXISTS idx_proxylist_proto ON proxylist(proto)')
+        # Migration: add latency columns if missing
+        _migrate_latency_columns(sqlite)
 
     elif dbname == 'uris':
         sqlite.execute("""CREATE TABLE IF NOT EXISTS uris (