docs: add pets subsystem reference

docs/architecture/subsystems/pets.md

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+# pets
+
+## Purpose
+
+Pet rule engine. Defines composable conditions (detected-in-zone, not-seen-for-N-minutes, detected-without-person, time-of-day, etc.) and per-pet state tracking that downstream code uses to trigger pet-specific alerts and automations. The on-camera pet identification itself lives in `vigilar/detection/pet_id.py`; this package is the behavioural layer on top of those sightings.
+
+## Key files
+
+- `vigilar/pets/rules.py` — `PetState` dataclass, `evaluate_condition`, rule evaluator; reads `pet_rules` via `get_all_enabled_rules`
+- `vigilar/pets/__init__.py` (empty)
+
+## MQTT topics
+
+**Subscribes:** No MQTT subscribers found at time of writing. The module is invoked as a library — typically from the events processor or an evaluator that already has topic payloads in hand.
+**Publishes:** No MQTT publishers found at time of writing.
+
+## Database tables
+
+- `pet_rules` — loaded via `vigilar.storage.queries.get_all_enabled_rules`
+- `pets`, `pet_sightings`, `pet_training_images` — owned schema-wise by this feature area but written from elsewhere (events processor for sightings, detection/trainer for training images, web for pets CRUD)
+
+## Depends on
+
+- `storage` — reads `pet_rules`
+- `alerts.profiles` — reuses `is_in_time_window` for time-of-day conditions
+
+## Consumed by
+
+- `events` / rule evaluator — drives pet-specific alert actions
+- `web` — the `pets` blueprint manages pets, training images, and rules
+
+## Notes
+
+`PetState` is per-pet, in-memory, and tracks last-seen camera and time, current zone with entry time, and whether a person is currently present alongside the pet. This makes rules like "cat in kitchen for more than 5 minutes while nobody is home" possible without joining SQLite on every frame. The condition schema is a plain dict with a `type` discriminator, so new condition kinds can be added without schema changes.