feat(backend): add C2Connector ABC + payload mapping + factory (B0.4)

- abstract C2Connector with authenticate / list_hosts / execute_task /
  get_task_result / cancel_task / execute_cleanup; stream_task_output
  optional v1 (NotImplementedError).
- Payload / TaskHandle / TaskResult / TaskStatus frozen dataclasses.
- UnsupportedPayloadType raised when no native command maps to the
  chosen (c2_type, payload_type) pair.
- Mythic payload_type → native command map populated (spec §7 table).
- HOME map left empty until PR2 is closed.
- ConnectorFactory: register_connector decorator + build(c2_type) that
  instantiates + authenticates via an injected config resolver.

No real Mythic / Home implementations land in this sprint.

backend/src/mimic/connectors/__init__.py

@@ -0,0 +1,27 @@
+"""C2Connector abstraction.
+
+Sprint 0 ships the interface + dataclasses + payload mapping + factory.
+Concrete `MythicConnector` and `HomeConnector` implementations land after
+PR1 and PR2 respectively.
+"""
+
+from mimic.connectors.base import (
+    C2Connector,
+    Payload,
+    TaskHandle,
+    TaskResult,
+    TaskStatus,
+    UnsupportedPayloadType,
+)
+from mimic.connectors.factory import ConnectorFactory, register_connector
+
+__all__ = [
+    "C2Connector",
+    "ConnectorFactory",
+    "Payload",
+    "TaskHandle",
+    "TaskResult",
+    "TaskStatus",
+    "UnsupportedPayloadType",
+    "register_connector",
+]