ci: add flake8 lint workflow; fix unused imports

Adds .gitea/workflows/lint.yml running flake8 with .flake8 config.
Removes unused imports (flask.redirect, flask.url_for, time, typing.Tuple).
Config ignores E221/E305 (intentional column alignment and function spacing).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -13,7 +13,7 @@ import time
 import uuid
 from functools import wraps
 
-from flask import Flask, jsonify, redirect, render_template, request, url_for
+from flask import Flask, jsonify, render_template, request
 
 import db
 import diagnose
@@ -42,6 +42,7 @@ def inject_config():
         }
     }
 
+
 # In-memory diagnostic job store { job_id: { status, result, created_at } }
 _diag_jobs: dict = {}
 _diag_lock = threading.Lock()