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/actions/__init__.py

@@ -2,4 +2,13 @@
 
 from __future__ import annotations
 
-__all__: list[str] = []
+from .store import all_actions, get_action, register_action
+from .types import ActionDefinition, ActionHandler
+
+__all__ = [
+    "ActionDefinition",
+    "ActionHandler",
+    "all_actions",
+    "get_action",
+    "register_action",
+]

surfsense_backend/app/automations/registries/actions/store.py

@@ -0,0 +1,33 @@
+"""Action registry: in-memory dict + ``register_action`` API."""
+
+from __future__ import annotations
+
+from .types import ActionDefinition
+
+_REGISTRY: dict[str, ActionDefinition] = {}
+
+
+def register_action(action: ActionDefinition) -> None:
+    """Add an action to the in-memory registry.
+
+    Raises ``ValueError`` on duplicate ``type`` — registration runs
+    once per process, so a duplicate is always a bug.
+    """
+
+    if action.type in _REGISTRY:
+        raise ValueError(
+            f"Action already registered: {action.type!r}"
+        )
+    _REGISTRY[action.type] = action
+
+
+def get_action(action_type: str) -> ActionDefinition | None:
+    """Look up one action by type. Returns ``None`` on miss."""
+
+    return _REGISTRY.get(action_type)
+
+
+def all_actions() -> dict[str, ActionDefinition]:
+    """Snapshot of the registry as a defensive copy."""
+
+    return dict(_REGISTRY)

surfsense_backend/app/automations/registries/actions/types.py

@@ -0,0 +1,44 @@
+"""``ActionDefinition`` dataclass — the v1-minimum action shape."""
+
+from __future__ import annotations
+
+from collections.abc import Awaitable, Callable
+from dataclasses import dataclass
+from typing import Any
+
+ActionHandler = Callable[[dict[str, Any]], Awaitable[Any]]
+"""The signature every action handler must satisfy.
+
+Identical in shape to ``CapabilityHandler`` — both receive a
+caller-validated input dict and return an arbitrary output. The
+distinction is purely architectural: capabilities are the low-level
+"what SurfSense can do" surface, actions are the user-facing
+building blocks composed into a plan.
+"""
+
+
+@dataclass(frozen=True, slots=True)
+class ActionDefinition:
+    """A user-facing step type the plan editor can compose.
+
+    v1 trims the dataclass to the five fields necessary for
+    registry dispatch and form rendering. The full design (§4)
+    includes ``output_contract``, ``uses_capabilities``, and
+    ``produces_artifacts``; all three are deferred until a consumer
+    feature requires them:
+
+    - ``output_contract`` — the loose ``agent_task`` action declares
+      its output shape per-step via ``config.output_schema``, so the
+      action-level contract is not needed in v1.
+    - ``uses_capabilities`` — would let the NL generator do static
+      analysis of which capabilities each action invokes; deferred
+      because v1 ships a single (``agent_task``) action.
+    - ``produces_artifacts`` — deferred alongside the artifact
+      pipeline (see §13 decision 26).
+    """
+
+    type: str
+    name: str
+    description: str
+    config_schema: dict[str, Any]
+    handler: ActionHandler