docs: add coding conventions reference

docs/architecture/conventions.md

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+# Coding Conventions
+
+These are the rules a contributor needs in their head before touching Vigilar.
+They are distilled from `CLAUDE.md` and from the patterns already in the codebase.
+
+## Language and style
+
+- Python 3.11+.
+- Ruff for linting. Line length 100 (see `pyproject.toml`).
+- Type hints on all public functions. Internal helpers may omit them when the
+  types are obvious from context.
+- No docstrings unless the logic is non-obvious. Short, well-named functions
+  should speak for themselves.
+
+## String constants
+
+All string constants referenced in more than one place live in
+`vigilar/constants.py`. The file uses `StrEnum` for enumerated values —
+`ArmState`, `Severity`, `EventType`, `SensorType`, `AlertChannel`,
+`RecordingTrigger`, and friends — and a `Topics` class for MQTT topic builders.
+
+If you are about to write `"MOTION_START"` or `"vigilar/camera/..."` in a second
+place, stop and add it to the appropriate enum or the `Topics` class instead.
+Defaults like `DEFAULT_IDLE_FPS` also live in this module.
+
+## Database access
+
+- SQLite in WAL mode, opened through `vigilar/storage/db.py`.
+- SQLAlchemy Core expressions only — no mapped classes, no ORM session.
+- Schema lives in `vigilar/storage/schema.py`. Named query helpers live in
+  `vigilar/storage/queries.py`.
+- New tables go in `schema.py`. New query helpers go in `queries.py`. Do not
+  scatter ad-hoc SQL across subsystem code.
+
+## Processes and the MQTT bus
+
+- Each subsystem runs in its own `multiprocessing.Process`, spawned by the
+  supervisor in `vigilar/main.py` via the `SubsystemProcess` wrapper. Cameras
+  are the exception: `CameraManager` owns one child process per camera
+  directly.
+- Subsystems communicate only through the local MQTT broker at
+  `127.0.0.1:1883`. No direct imports or function calls across subsystem
+  boundaries.
+- Topic naming: `vigilar/<domain>/<id>/<event>`. Every topic comes from the
+  `Topics` class in `vigilar/constants.py` — do not construct topic strings
+  ad hoc.
+- If you are tempted to reach into another subsystem directly, publish a
+  topic instead.
+
+## Configuration
+
+- `config/vigilar.toml` is the only config file the app reads at runtime.
+- It is validated by Pydantic v2 models in `vigilar/config.py`. Add new fields
+  to the Pydantic models, not just to the TOML.
+- Secrets are file paths, not inline values. Never put a key, password, or
+  token directly in the TOML.
+
+## Testing
+
+- `pytest` from the repo root.
+- Tests live under `tests/`.
+
+## Committing
+
+- `ruff check vigilar/` must pass.
+- `pytest` must pass.
+- One logical change per commit. Commit often.