feat(hosting): signed preview URLs for POC sites

Replace the standalone POC viewer (duplicate rendering) with signed
JWT preview tokens that bypass StorefrontAccessMiddleware:

Architecture:
1. Admin clicks Preview → route generates signed JWT
2. Redirects to /storefront/{subdomain}/homepage?_preview=token
3. Middleware validates token signature + expiry + store_id
4. Sets request.state.is_preview = True, skips subscription check
5. Full storefront renders with HostWizard preview banner injected

New files:
- app/core/preview_token.py: create_preview_token/verify_preview_token

Changes:
- middleware/storefront_access.py: preview token bypass before sub check
- storefront/base.html: preview banner injection via is_preview state
- hosting/routes/pages/public.py: redirect with signed token (was direct render)
- hosting/routes/api/admin_sites.py: GET /sites/{id}/preview-url endpoint

Removed:
- hosting/templates/hosting/public/poc-viewer.html (replaced by storefront)

Benefits: one rendering path, all section types work, shareable 24h links.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/core/preview_token.py

@@ -0,0 +1,54 @@
+# app/core/preview_token.py
+"""
+Signed preview tokens for POC site previews.
+
+Generates time-limited JWT tokens that allow viewing storefront pages
+for stores without active subscriptions (POC sites). The token is
+validated by StorefrontAccessMiddleware to bypass the subscription gate.
+"""
+
+import logging
+from datetime import UTC, datetime, timedelta
+
+from jose import JWTError, jwt
+
+from app.core.config import settings
+
+logger = logging.getLogger(__name__)
+
+PREVIEW_TOKEN_HOURS = 24
+ALGORITHM = "HS256"
+
+
+def create_preview_token(store_id: int, store_code: str, site_id: int) -> str:
+    """Create a signed preview token for a POC site.
+
+    Token is valid for PREVIEW_TOKEN_HOURS (default 24h) and is tied
+    to a specific store_id. Shareable with clients for preview access.
+    """
+    payload = {
+        "sub": f"preview:{store_id}",
+        "store_id": store_id,
+        "store_code": store_code,
+        "site_id": site_id,
+        "preview": True,
+        "exp": datetime.now(UTC) + timedelta(hours=PREVIEW_TOKEN_HOURS),
+        "iat": datetime.now(UTC),
+    }
+    return jwt.encode(payload, settings.jwt_secret_key, algorithm=ALGORITHM)
+
+
+def verify_preview_token(token: str, store_id: int) -> bool:
+    """Verify a preview token is valid and matches the store.
+
+    Returns True if:
+    - Token signature is valid
+    - Token has not expired
+    - Token has preview=True claim
+    - Token store_id matches the requested store
+    """
+    try:
+        payload = jwt.decode(token, settings.jwt_secret_key, algorithms=[ALGORITHM])
+        return payload.get("preview") is True and payload.get("store_id") == store_id
+    except JWTError:
+        return False