feat: add multi-server support

Connect to multiple IRC servers concurrently from a single config file.
Plugins are loaded once and shared; per-server state is isolated via
separate SQLite databases and per-bot runtime state (bot._pstate).

- Add build_server_configs() for [servers.*] config layout
- Bot.__init__ gains name parameter, _pstate dict for plugin isolation
- cli.py runs multiple bots via asyncio.gather
- 9 stateful plugins migrated from module-level dicts to _ps(bot) pattern
- Backward compatible: legacy [server] config works unchanged

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_paste_overflow.py

@@ -29,7 +29,7 @@ def _make_bot(*, paste_threshold: int = 4, flaskpaste_mod=None) -> Bot:
     registry = PluginRegistry()
     if flaskpaste_mod is not None:
         registry._modules["flaskpaste"] = flaskpaste_mod
-    bot = Bot(config, registry)
+    bot = Bot("test", config, registry)
     bot._sent: list[tuple[str, str]] = []  # type: ignore[attr-defined]
 
     async def _capturing_send(target: str, text: str) -> None: