feat(automation): add empty Capability / Action / Trigger registries

Three registries under app/automations/registries/, each as its own
folder with the same SRP-per-file split (types.py for the dataclass,
store.py for the in-memory dict + register/get/all functions). All
three start empty; concrete entries land when the user signs off on
which capabilities / actions / triggers to include (step 2).

Capability (locked at v1-minimum five fields — see commit 2):
  - id, description, input_schema, output_schema, handler
  - CapabilityHandler = Callable[[dict[str, Any]], Awaitable[Any]]
  - Frozen, slotted dataclass (immutable post-registration).

ActionDefinition (v1-trim of design plan §4):
  - type, name, description, config_schema, handler
  - Defers output_contract (handled per-step by agent_task's
    config.output_schema), uses_capabilities (no static analysis
    needed until >1 action ships), and produces_artifacts (deferred
    alongside the artifact pipeline).

TriggerDefinition (declarative, no handler):
  - type, description, config_schema, payload_schema
  - No handler field — firing is a single dispatcher's
    responsibility, not a per-trigger one.

store.py contract for all three:
  - register_*: idempotent at process startup, raises on duplicate
  - get_*: returns None on miss
  - all_*: returns a defensive copy of the registry dict

Verified by an inline smoke test (10 checks): empty initial state,
registration and lookup work, duplicates raise, frozen dataclasses
reject mutation, snapshots are copies, handlers are awaitable.

Isolation invariant audit: grep across the full app/automations/
tree shows only three app.* imports, all of them
``from app.db import BaseModel, TimestampMixin`` in the model files.
No imports from app.agents.*, app.services.*, app.tasks.*,
app.routes.*, or any other business-logic module.

surfsense_backend/app/automations/registries/__init__.py

@@ -2,4 +2,40 @@
 
 from __future__ import annotations
 
-__all__: list[str] = []
+from .actions import (
+    ActionDefinition,
+    ActionHandler,
+    all_actions,
+    get_action,
+    register_action,
+)
+from .capabilities import (
+    Capability,
+    CapabilityHandler,
+    all_capabilities,
+    get_capability,
+    register_capability,
+)
+from .triggers import (
+    TriggerDefinition,
+    all_triggers,
+    get_trigger,
+    register_trigger,
+)
+
+__all__ = [
+    "ActionDefinition",
+    "ActionHandler",
+    "Capability",
+    "CapabilityHandler",
+    "TriggerDefinition",
+    "all_actions",
+    "all_capabilities",
+    "all_triggers",
+    "get_action",
+    "get_capability",
+    "get_trigger",
+    "register_action",
+    "register_capability",
+    "register_trigger",
+]