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/factory.py

@@ -0,0 +1,55 @@
+"""Connector factory keyed on `c2_type`.
+
+Concrete connectors register themselves at import time via the
+`@register_connector` decorator. Sprint 0 ships only the interface — no real
+implementation registers in this codebase yet.
+"""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+from typing import TypeVar
+
+from mimic.connectors.base import C2Connector
+from mimic.db.types import C2Type
+
+ConnectorClass = type[C2Connector]
+C = TypeVar("C", bound=C2Connector)
+
+_REGISTRY: dict[C2Type, ConnectorClass] = {}
+
+
+def register_connector(c2_type: C2Type) -> Callable[[type[C]], type[C]]:
+    """Class decorator: register a concrete connector under its C2Type."""
+
+    def _wrap(klass: type[C]) -> type[C]:
+        if c2_type in _REGISTRY:
+            raise RuntimeError(f"connector already registered for {c2_type.value}")
+        _REGISTRY[c2_type] = klass
+        klass.name = c2_type
+        return klass
+
+    return _wrap
+
+
+class ConnectorFactory:
+    """Resolves a connector instance for a given C2 type."""
+
+    def __init__(self, config_resolver: Callable[[C2Type], dict[str, object]]):
+        self._resolver = config_resolver
+
+    @staticmethod
+    def registered() -> dict[C2Type, ConnectorClass]:
+        return dict(_REGISTRY)
+
+    def build(self, c2_type: C2Type) -> C2Connector:
+        try:
+            klass = _REGISTRY[c2_type]
+        except KeyError as exc:
+            raise NotImplementedError(
+                f"no connector registered for {c2_type.value}"
+            ) from exc
+
+        connector = klass()
+        connector.authenticate(self._resolver(c2_type))
+        return connector