fix(marketing): RFC 9309 robots.txt semantics + lazy marketing import

- Remove redundant named User-agent blocks (Googlebot, Bingbot, ClaudeBot,
  GPTBot, PerplexityBot, Applebot) that per RFC 9309 §2.2 overrode the
  wildcard and granted those bots access to /api/, /admin, /account.
- Add explicit Google-Extended and ChatGPT-User blocks (AI opt-in
  signaling) with full Allow/Disallow rule sets.
- Fix /blog → /blog/ for prefix-match consistency.
- Move src.marketing.routes import inside recordings.index() function
  to localize cross-blueprint dependency (was at module top, inverting
  initialization order).
- Add shadow-warning comment at marketing_bp registration site.

src/api/recordings.py

@@ -39,7 +39,6 @@ from src.file_exporter import export_recording, mark_export_as_deleted
 from src.utils.ffprobe import get_codec_info, get_creation_date, FFProbeError
 from src.utils.audio_conversion import convert_if_needed
 from src.utils.file_hash import compute_file_sha256
-from src.marketing.routes import landing as _marketing_landing
 
 # Create blueprint
 recordings_bp = Blueprint('recordings', __name__)
@@ -1374,8 +1373,14 @@ def index():
     endpoints are mounted at "/". Since recordings_bp is registered first,
     Flask's URL map resolves "/" to recordings.index, so a redirect would
     loop back into this same handler indefinitely.
+
+    The src.marketing.routes import is lazy (inside the function) on
+    purpose: it localizes the cross-blueprint dependency to the call
+    site rather than coupling recordings_bp module load to marketing_bp
+    module load, preserving the apparent initialization order in app.py.
     """
     if not current_user.is_authenticated:
+        from src.marketing.routes import landing as _marketing_landing
         return _marketing_landing()
 
     # Check if user is a group admin