docs: update AGENTS.md with new guidelines

- Add ui-ux-pro-max skill reference
- Add keyboard overlay behavior rules for Flutter
- Update testing strategy to prioritize high-regression flows
- Add copy priority rules for auth and form pages
- Update minimal interface copy guidelines

backend/AGENTS.md

@@ -164,6 +164,20 @@ Use `schemas / repository / service` pattern:
 - `service.py` — authorization + business logic + transaction boundary (must commit/rollback)
 - `dependencies.py` — DI (`get_db`, `get_current_user`)
 
+### Schema-First and Strong Typing (Mandatory)
+
+**Data model constraints are the first priority. Define schemas before implementation.**
+
+- Any backend feature that introduces or changes data structures MUST define/update strong-typed schemas first.
+- All request/response/domain/runtime contracts MUST use explicit Pydantic models or typed dataclasses.
+- Prohibit weak typing in data contracts: `Any`, untyped `dict`, untyped `list`, `object` placeholders.
+- Prohibit using raw `dict[str, object]` as the canonical contract for pipeline/stage/config/domain payloads.
+- External library boundaries may accept weakly typed input only at adapter edges; data MUST be converted immediately into local strong-typed schemas before entering service/domain layers.
+- New model placement rules:
+  - Cross-module runtime/domain contracts: `backend/src/schemas/**`
+  - HTTP request/response contracts: `backend/src/v1/**/schemas.py`
+  - ORM persistence models: `backend/src/models/**`
+
 ### Auth & Data Access
 
 - Backend must verify JWT signature and expiration (not just decode)