Fix 404 on the header logo caused by an encoded query string

base.html passed the cache-busting query string INSIDE the filename argument:

    url_for('static', filename='uploads/header_logo.png?v=1')

Flask percent-encodes the '?' to '%3F', producing a request for a file
literally named "header_logo.png%3Fv=1", which does not exist. Every
authenticated page therefore logged:

    GET /static/uploads/header_logo.png%3Fv=1 404

The logo silently stayed hidden too, because the element carries
onerror="this.style.display='none'".

Changes:
- base.html / login.html: append "?v=..." OUTSIDE url_for(), matching what
  admin/customize_logos.html already did correctly.
- Inject logo_version from a context processor instead of hardcoding it.
  It is derived from the newest logo's mtime, so uploading a new logo in
  Admin -> Customize Logos is reflected immediately. The previous value was
  a literal 1, which defeated cache-busting entirely; login.html used
  range(1, 999999) | random, which changed on every request and made the
  logo uncacheable.

Verified: /, /content/ and /players/ now render
/static/uploads/header_logo.png?v=<mtime> (HTTP 200), and no %3F requests
appear in the logs.

app/app.py

@@ -191,9 +191,29 @@ def register_context_processors(app):
     @app.context_processor
     def inject_config():
         """Inject configuration variables into all templates"""
+        # Cache-busting token for the uploadable logos.
+        #
+        # Must be appended OUTSIDE url_for(): passing 'logo.png?v=1' as the
+        # filename makes Flask percent-encode the '?' into '%3F', which asks
+        # for a file literally named "logo.png%3Fv=1" and 404s.
+        #
+        # Derived from the newest logo's mtime so that uploading a new logo in
+        # Admin → Customize Logos is picked up immediately, rather than reusing
+        # a hardcoded value the browser has already cached.
+        logo_version = 1
+        for _logo_name in ('header_logo.png', 'login_logo.png'):
+            try:
+                _mtime = int(os.path.getmtime(
+                    os.path.join(app.config['UPLOAD_FOLDER'], _logo_name)))
+                logo_version = max(logo_version, _mtime)
+            except OSError:
+                # Logo not uploaded yet — keep the current token.
+                pass
+
         return {
             'server_version': app.config['SERVER_VERSION'],
             'build_date': app.config['BUILD_DATE'],
+            'logo_version': logo_version,
             'logo_exists': os.path.exists(
                 os.path.join(app.root_path, app.config['UPLOAD_FOLDERLOGO'], 'logo.png')
             )