feat: add e2e test suite with Robot Framework

Add e2e/ dir with Robot Framework tests for page loading, ban records,
blocklist import, config edit. Add requirements.txt. Update Makefile with
test commands. Update .gitignore, backend docs, testing requirements docs.

e2e/resources/auth.resource

@@ -0,0 +1,31 @@
+*** Settings ***
+Resource    ${CURDIR}/common.resource
+
+*** Keywords ***
+Login As Admin
+    # Check setup status.
+    ${response}=    GET    ${BACKEND_URL}/api/setup/status
+    ${body}=    Set Variable    ${response.json()}
+
+    IF    ${body}[setup_complete] == ${false}
+        # Complete the setup wizard with the dev master password ("Hallo123!").
+        ${password}=    Set Variable    Hallo123!
+        ${hashed}=    Evaluate    "sha256('${password}'.encode()).hexdigest()"    modules=hashlib
+        ${setup_payload}=    Create Dictionary    password=${hashed}
+        POST    ${BACKEND_URL}/api/setup/complete    json=${setup_payload}
+
+        # Retry login after setup.
+        ${response}=    GET    ${BACKEND_URL}/api/auth/login
+    END
+
+    # Perform login.
+    ${password}=    Set Variable    Hallo123!
+    ${hashed}=    Evaluate    "sha256('${password}'.encode()).hexdigest()"    modules=hashlib
+    ${login_payload}=    Create Dictionary    password=${hashed}
+    ${response}=    POST    ${BACKEND_URL}/api/auth/login    json=${login_payload}
+
+    # Store session cookie for subsequent requests.
+    ${cookies}=    Get Cookies
+    ${session_cookie}=    Get Cookie Value    bangui_session
+    Set Browser Variables    session_cookie=${session_cookie}
+    Log    Logged in as admin.

e2e/resources/common.resource

@@ -0,0 +1,25 @@
+*** Settings ***
+Library    Browser
+Library    HttpLibrary
+
+Variables    ${CURDIR}/../../.env
+
+# Health check timeout for suite setup (120 s poll interval).
+Suite Setup    Wait For Backend Health    timeout=120    interval=5
+
+*** Variables ***
+${FRONTEND_URL}    http://localhost:5173
+${BACKEND_URL}     http://localhost:8000
+
+*** Keywords ***
+Wait For Backend Health
+    [Arguments]    ${timeout}=120    ${interval}=5
+    ${deadline}=    Evaluate    time.time() + ${timeout}
+    WHILE    True
+        ${now}=    Evaluate    time.time()
+        IF    ${now} >= ${deadline}    FAIL    Backend did not become healthy within ${timeout} seconds
+        ${response}=    GET    ${BACKEND_URL}/api/health    expected_status=200
+        IF    ${response.status} == 200    BREAK
+        Sleep    ${interval}
+    END
+    Log    Backend is healthy.