M1-T07: align docs to single-DB reality and re-export OpenAPI

Rewrite README (single app.db + one alembic_app chain, legacy data moved
once via scripts.migrate_legacy_data, accurate test list) and remove the
Grafana Provisioning section. Update architecture-overview to the unified
data layer (one Base, app-DB engine with WAL) and retire the
alembic_location / alembic_poo sections. Mark M1 done in the roadmap.
Re-export openapi/, which catches the spec up to the already-existing
/config/smtp/test and /public-ip/check endpoints (purely additive; M1's
DB-session dependency swap produced no schema change).

pytest 95 passed; ruff clean (pre-existing only); OpenAPI export idempotent.

docs/architecture-overview.md

@@ -23,10 +23,8 @@
   - 基础路由注册
 - `config.py`
   - 环境变量驱动的 settings
-- `auth_db.py`
-  - app 级共享 auth 数据库
 - `db.py`
-  - SQLAlchemy engine / session / Base
+  - 统一数据层：一个 `Base`、一个绑定 `app_database_url` 的 cached engine（SQLite WAL）、`get_engine` / `get_session_local` / `reset_db_caches` / `get_db_session`
 - `dependencies.py`
   - 通用依赖注入
 - `api/`
@@ -37,7 +35,7 @@
   - 当前已迁入 `POST /poo/record` 与 `GET /poo/latest`
 - `models/`
   - SQLAlchemy models
-  - 当前 `auth`、`location` 与 `poo` 使用各自独立的数据库 base
+  - 所有模型（auth / config / public_ip / location / poo）共用同一个 `Base`，均落在单一 `app.db` 中
 - `schemas/`
   - Pydantic schemas
 - `services/`
@@ -53,17 +51,9 @@
 - `static/`
   - 极简静态资源
 
-### `alembic_location/`
-
-Location DB 的 migration 基础设施。
-
 ### `alembic_app/`
 
-App DB 的 migration 基础设施。
-
-### `alembic_poo/`
-
-Poo DB 的 migration 基础设施。
+App DB 的唯一 Alembic migration 链，同时管理 `location` / `poo_records` 表。M1 将三个独立 DB 合并进 `app.db` 后，`alembic_location/` 与 `alembic_poo/` 已退役，全部由此链统一管理。
 
 ### `tests/`