Add tutorials/03-server-modules-security.md

Feat(tutorials): add stubs for 03 Security (RBAC)

tutorials/03-server-modules-security.md

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+**`tutorials/03-server-modules-security.md`**
+
+````markdown
+# 03 — Server Modules & Security
+
+## Roles & Helper
+```python
+import anvil.server
+from anvil import users, secrets
+
+ROLES = {"viewer","editor","admin"}
+
+def require_role(*roles):
+  u = users.get_user()
+  if not u: raise PermissionError("Login required")
+  if not set(roles).intersection(set(u.get('roles', []))):
+    raise PermissionError("Insufficient role")
+````
+
+## Validated Callable
+
+```python
+@anvil.server.callable
+def secure_greet(name):
+  name = (name or "").strip()
+  if not (1 <= len(name) <= 64): raise ValueError("Bad name")
+  return f"Top-secret hello, {name}"
+```
+
+## Secrets + Fake External
+
+```python
+API_KEY = secrets.get_secret('DEMO_API_KEY')  # set in Anvil
+```
+
+## Simple Rate Limit (per user)
+
+Use a module-level dict `{user_id: [timestamps...]}`; trim to last 60s, allow ≤5.
+
+## Client UX
+
+* Try/catch PermissionError → Notification("You need editor role").
+* Disable button while awaiting server.
+
+````
+
+**`tutorials/04-uplink-file-hash.md`**
+```markdown
+# 04 — Uplink: Local File Hash
+
+## Uplink Script (local)
+```python
+import anvil.server, hashlib
+@anvil.server.callable("hash_file")
+def hash_file(buffer):
+  h = hashlib.sha256(buffer.get_bytes())
+  return h.hexdigest()
+anvil.server.connect("YOUR-UPLINK-KEY")
+anvil.server.wait_forever()
+````
+
+## Client
+
+* FileLoader → `file_loader_1_change`: call `hash_file` with the Media object.
+* Show result; if timeout/conn error → “Uplink offline”.
+
+````
+
+**`tutorials/05-auth-roles-ui.md`**
+```markdown
+# 05 — Auth: Users + Roles + UI Gating
+
+## Setup
+Enable Users service. Add roles to users (e.g., `editor`, `admin`).
+
+## Server
+Use `require_role` from Tutorial 03 to gate `dangerous_action`.
+
+## Client
+- Add Login/Logout buttons.
+- Show current user + roles.
+- Hide/show editor-only actions via `component.visible`.
+
+## Demo Flow
+- Viewer logs in → can’t see editor actions.
+- Editor logs in → sees & runs gated action.
+````