phase 2: code cleanup and simplification

- Remove unused result_queue from WorkerThread and worker mode
- Remove num_targets abstraction, simplify to single-target mode
- Add _db_context() context manager for database connections
- Refactor 5 call sites to use context manager (finish, init, cleanup_stale, periodic saves)
- Mark _prep_db/_close_db as deprecated
- Add __version__ = '2.0.0' to ppf.py
- Add thread spawn stagger (0-100ms) in worker mode for Tor-friendly startup

ppf.py

@@ -1,5 +1,7 @@
 #!/usr/bin/env python2
 
+__version__ = '2.0.0'
+
 import sys
 import os
 
@@ -481,13 +483,12 @@ def worker_main(config):
 
     # Create shared queues for worker threads
     job_queue = proxywatchd.PriorityJobQueue()
-    result_queue = Queue.Queue()
     completion_queue = Queue.Queue()
 
     # Spawn worker threads with stagger to avoid overwhelming Tor
     threads = []
     for i in range(num_threads):
-        wt = proxywatchd.WorkerThread('w%d' % i, job_queue, result_queue)
+        wt = proxywatchd.WorkerThread('w%d' % i, job_queue)
         wt.start_thread()
         threads.append(wt)
         time.sleep(random.random() / 10)  # 0-100ms stagger per thread