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>

migrations/0002-event-tables.sql

@@ -0,0 +1,56 @@
+-- ABOUTME: Creates event sourcing tables (events, event_revisions, event_tombstones).
+-- ABOUTME: Also creates idempotency_nonces and event_animals tables.
+
+-- events: Core event log table
+CREATE TABLE events (
+    id TEXT PRIMARY KEY CHECK(length(id) = 26),
+    type TEXT NOT NULL,
+    ts_utc INTEGER NOT NULL,
+    actor TEXT NOT NULL,
+    entity_refs TEXT NOT NULL CHECK(json_valid(entity_refs)),
+    payload TEXT NOT NULL CHECK(json_valid(payload)),
+    version INTEGER NOT NULL DEFAULT 1
+);
+CREATE INDEX idx_events_ts ON events(ts_utc);
+CREATE INDEX idx_events_type_ts ON events(type, ts_utc);
+CREATE INDEX idx_events_actor_ts ON events(actor, ts_utc);
+
+-- event_revisions: Stores prior versions when events are edited
+CREATE TABLE event_revisions (
+    event_id TEXT NOT NULL CHECK(length(event_id) = 26),
+    version INTEGER NOT NULL,
+    ts_utc INTEGER NOT NULL,
+    actor TEXT NOT NULL,
+    entity_refs TEXT NOT NULL CHECK(json_valid(entity_refs)),
+    payload TEXT NOT NULL CHECK(json_valid(payload)),
+    edited_at_utc INTEGER NOT NULL,
+    edited_by TEXT NOT NULL,
+    PRIMARY KEY (event_id, version)
+);
+
+-- event_tombstones: Immutable deletion records
+CREATE TABLE event_tombstones (
+    id TEXT PRIMARY KEY CHECK(length(id) = 26),
+    ts_utc INTEGER NOT NULL,
+    actor TEXT NOT NULL,
+    target_event_id TEXT NOT NULL CHECK(length(target_event_id) = 26),
+    reason TEXT
+);
+CREATE INDEX idx_event_tombstones_target ON event_tombstones(target_event_id);
+
+-- idempotency_nonces: Prevents duplicate POST submissions
+CREATE TABLE idempotency_nonces (
+    nonce TEXT PRIMARY KEY,
+    actor TEXT NOT NULL,
+    route TEXT NOT NULL,
+    created_at_utc INTEGER NOT NULL
+);
+
+-- event_animals: Links events to affected animals
+CREATE TABLE event_animals (
+    event_id TEXT NOT NULL CHECK(length(event_id) = 26),
+    animal_id TEXT NOT NULL CHECK(length(animal_id) = 26),
+    ts_utc INTEGER NOT NULL,
+    PRIMARY KEY (event_id, animal_id)
+);
+CREATE UNIQUE INDEX ux_event_animals_animal_ts ON event_animals(animal_id, ts_utc);