feat: add event tables schema and models

Create migration for events, event_revisions, event_tombstones,
idempotency_nonces, and event_animals tables with ULID checks
and JSON validation. Add Pydantic models with field validators.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/animaltrack/models/__init__.py

@@ -1,6 +1,13 @@
 # ABOUTME: Models package for AnimalTrack.
-# ABOUTME: Exports all Pydantic models and enums for reference data.
+# ABOUTME: Exports all Pydantic models and enums for reference and event data.
 
+from animaltrack.models.events import (
+    Event,
+    EventAnimal,
+    EventRevision,
+    EventTombstone,
+    IdempotencyNonce,
+)
 from animaltrack.models.reference import (
     FeedType,
     Location,
@@ -12,7 +19,12 @@ from animaltrack.models.reference import (
 )
 
 __all__ = [
+    "Event",
+    "EventAnimal",
+    "EventRevision",
+    "EventTombstone",
     "FeedType",
+    "IdempotencyNonce",
     "Location",
     "Product",
     "ProductUnit",

src/animaltrack/models/events.py

@@ -0,0 +1,92 @@
+# ABOUTME: Pydantic models for event sourcing tables.
+# ABOUTME: Includes Event, EventRevision, EventTombstone, IdempotencyNonce, EventAnimal.
+
+from pydantic import BaseModel, Field, field_validator
+
+
+class Event(BaseModel):
+    """Core event model representing a state change in the system."""
+
+    id: str = Field(..., min_length=26, max_length=26)
+    type: str
+    ts_utc: int
+    actor: str
+    entity_refs: dict
+    payload: dict
+    version: int = 1
+
+    @field_validator("id")
+    @classmethod
+    def id_must_be_26_chars(cls, v: str) -> str:
+        """Event ID must be exactly 26 characters (ULID format)."""
+        if len(v) != 26:
+            msg = "Event ID must be exactly 26 characters"
+            raise ValueError(msg)
+        return v
+
+
+class EventRevision(BaseModel):
+    """Stores prior versions of events when they are edited."""
+
+    event_id: str = Field(..., min_length=26, max_length=26)
+    version: int
+    ts_utc: int
+    actor: str
+    entity_refs: dict
+    payload: dict
+    edited_at_utc: int
+    edited_by: str
+
+    @field_validator("event_id")
+    @classmethod
+    def event_id_must_be_26_chars(cls, v: str) -> str:
+        """Event ID must be exactly 26 characters (ULID format)."""
+        if len(v) != 26:
+            msg = "Event ID must be exactly 26 characters"
+            raise ValueError(msg)
+        return v
+
+
+class EventTombstone(BaseModel):
+    """Immutable record of event deletion."""
+
+    id: str = Field(..., min_length=26, max_length=26)
+    ts_utc: int
+    actor: str
+    target_event_id: str = Field(..., min_length=26, max_length=26)
+    reason: str | None = None
+
+    @field_validator("id", "target_event_id")
+    @classmethod
+    def ulid_must_be_26_chars(cls, v: str) -> str:
+        """ULID fields must be exactly 26 characters."""
+        if len(v) != 26:
+            msg = "ULID must be exactly 26 characters"
+            raise ValueError(msg)
+        return v
+
+
+class IdempotencyNonce(BaseModel):
+    """Tracks POST form nonces to prevent duplicate submissions."""
+
+    nonce: str
+    actor: str
+    route: str
+    created_at_utc: int
+
+
+class EventAnimal(BaseModel):
+    """Links events to affected animals for efficient querying."""
+
+    event_id: str = Field(..., min_length=26, max_length=26)
+    animal_id: str = Field(..., min_length=26, max_length=26)
+    ts_utc: int
+
+    @field_validator("event_id", "animal_id")
+    @classmethod
+    def ulid_must_be_26_chars(cls, v: str) -> str:
+        """ULID fields must be exactly 26 characters."""
+        if len(v) != 26:
+            msg = "ULID must be exactly 26 characters"
+            raise ValueError(msg)
+        return v