♻️(backend) refactor analytics backend from Protocol to abstract class

Switch the analytics backend interface from a typing Protocol to an
abstract base class, so the contract that backends must implement
is explicit and enforced at instantiation time.

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to
 - ⬆️(dependencies) update python dependencies
 - 💥(summary) remove v1 related code #1362
 - ✨(meet) use compatible with summary v2 #1362
+- ♻️(backend) refactor analytics backend from Protocol to abstract class
 
 ### Fixed
 

src/backend/core/analytics/base.py

@@ -1,12 +1,13 @@
 """Analytics backend protocol and default no-op implementation."""
 
-from typing import Any, Protocol
+from abc import ABC, abstractmethod
+from typing import Any
 
 from ..models import User
 from .events import AnalyticsEvent
 
 
-class AnalyticsBackend(Protocol):
+class AnalyticsBackend(ABC):
     """
     Interface every analytics backend must implement.
 
@@ -17,11 +18,11 @@ class AnalyticsBackend(Protocol):
         ANALYTICS_BACKEND_SETTINGS = {"api_key": "...", "host": "..."}
     """
 
-    def __init__(self, **kwargs: Any) -> None: ...
-
+    @abstractmethod
     def identify(self, user: User, properties: dict[str, Any] | None = None) -> None:
         """Associate traits (email, name, ...) with an identified user."""
 
+    @abstractmethod
     def capture(
         self,
         user: User,
@@ -30,16 +31,14 @@ class AnalyticsBackend(Protocol):
     ) -> None:
         """Record an event performed by an identified user."""
 
+    @abstractmethod
     def shutdown(self) -> None:
         """Flush pending events. Called on process exit."""
 
 
-class NoOpAnalytics:
+class NoOpAnalytics(AnalyticsBackend):
     """Default backend: silently discards everything."""
 
-    def __init__(self, **kwargs: Any) -> None:
-        """No-op: accepts and ignores any backend settings kwargs."""
-
     def identify(self, user: User, properties=None) -> None:
         """No-op: discards identify calls."""
 

src/backend/core/analytics/posthog.py

@@ -6,12 +6,13 @@ from typing import Any
 from posthog import Posthog
 
 from ..models import User
+from . import AnalyticsBackend
 from .events import AnalyticsEvent
 
 logger = logging.getLogger(__name__)
 
 
-class PostHogAnalytics:
+class PostHogAnalytics(AnalyticsBackend):
     """Send events to PostHog, keyed on the user's primary key (UUID)."""
 
     def __init__(