feat(automation): add Pydantic schemas for the automation definition

Three layers of Pydantic models under app/automations/schemas/, one
file per concern (SRP), matching the envelope in
automation-design-plan.md §5.

definition/ — the editable envelope persisted in
automations.definition:
  - envelope.py       AutomationDefinition (top-level shape)
  - plan_step.py      PlanStep (one step in the sequential plan)
  - inputs.py         InputsBlock (the inputs JSON Schema wrapper)
  - execution.py      ExecutionBlock (timeouts, retries, concurrency,
                                      budget cap, on_failure plan)
  - metadata.py       MetadataBlock (tags + created_from_nl + extras)
  - trigger_spec.py   TriggerSpec (one entry in triggers[])

triggers/ — per-trigger config schemas, dispatched by registry on the
TriggerSpec.type discriminator:
  - schedule.py       ScheduleTriggerConfig(cron, timezone)
  - manual.py         ManualTriggerConfig() — empty in v1

actions/ — per-action config schemas, dispatched by registry on the
PlanStep.action discriminator:
  - agent_task.py     AgentTaskActionConfig(prompt, tools, model,
                                            output_schema)

Design properties verified by an inline smoke test:
  - The §5 worked example round-trips through model_validate_json /
    model_dump_json byte-for-byte (InputsBlock uses
    serialize_by_alias so the JSON key stays "schema" not
    "schema_").
  - Envelope rejects unknown top-level keys (extra="forbid").
  - MetadataBlock tolerates unknown keys (extra="allow").
  - ExecutionBlock defaults apply when the block is omitted.
  - retry_backoff and concurrency are typed as Literal — bogus
    values rejected at validation time.
  - Per-type configs enforce their required fields (cron + timezone
    on schedule; non-empty prompt on agent_task).

The envelope keeps trigger and action configs as untyped dicts on
purpose — per-type validation is a registry-driven dispatch (commit
10), keeping the envelope free of every-type-knows-every-type
coupling.

surfsense_backend/app/automations/schemas/triggers/schedule.py

@@ -0,0 +1,33 @@
+"""``ScheduleTriggerConfig`` — config for the ``schedule`` trigger type."""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+class ScheduleTriggerConfig(BaseModel):
+    """Config for a cron-driven trigger.
+
+    Validated against ``AutomationTrigger.config`` whenever the
+    persisted ``type`` is ``schedule``. The cron expression is
+    evaluated by Celery Beat's source; the timezone is an IANA name
+    (e.g., ``Africa/Kigali``) and is required so the user's cron is
+    unambiguous across DST boundaries.
+    """
+
+    model_config = ConfigDict(extra="forbid")
+
+    cron: str = Field(
+        ...,
+        description=(
+            "Five-field cron expression. Minimum resolution is one "
+            "minute; the form editor warns when intervals tighter "
+            "than 15 minutes are used."
+        ),
+        examples=["0 9 * * 1-5"],
+    )
+    timezone: str = Field(
+        ...,
+        description="IANA timezone name (e.g., 'Africa/Kigali', 'UTC').",
+        examples=["Africa/Kigali"],
+    )