docs: stop claiming pixiv, and stop claiming everything gallery-dl supports (406 step 4)

Ships with the switch-off rather than with the code removal: a doc that promises a platform the product refuses is the Install and Public Surface area's characteristic defect.

pixiv comes out of README (twice), SECURITY.md (twice), .env.example and the compose header. The stored-credential warnings now name Patreon and SubscribeStar - the accounts that usually carry a payment method.

One correction beyond pixiv. README said FabledCurator follows creators on Patreon, SubscribeStar, Pixiv "and anything gallery-dl supports". That was already false: a platform not in the registry is rejected, however capable gallery-dl is. It now names the real set, which rule 171 records: Patreon, SubscribeStar, Discord and HentaiFoundry.

The 3422 docs guards still hold - the key path and bootstrap variable are untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SHQB1YukL3VyvMK8rcbmV9

README.md

@@ -16,9 +16,9 @@ through afterwards.
   artist, tag, post and series. A newest-first feed of what just arrived as the
   front page, a random Showcase, a filterable gallery, a similarity-driven
   Explore view, and a page-turning reader for series.
-- **Subscriptions.** Follows creators on Patreon, SubscribeStar, Pixiv and
-  anything `gallery-dl` supports, on a schedule. Handles paywalled posts using
-  your own logged-in session.
+- **Subscriptions.** Follows creators on Patreon, SubscribeStar, Discord and
+  HentaiFoundry, on a schedule. Handles paywalled posts using your own
+  logged-in session.
 - **ML tagging.** Runs image models in-container to suggest tags, group
   characters, find near-duplicates and power similarity search. Suggestions are
   reviewable — it proposes, you confirm, and it learns which proposals you keep
@@ -39,8 +39,8 @@ is no config file to edit beyond a handful of bootstrap environment variables.
 permission model. Anything that can reach the port is an administrator.
 
 That matters more here than it would in most self-hosted apps, because of what
-this one stores: **live platform session cookies for Patreon, SubscribeStar and
-Pixiv** — accounts that usually have a payment method attached. Whoever reaches
+this one stores: **live platform session cookies for Patreon and
+SubscribeStar** — accounts that usually have a payment method attached. Whoever reaches
 the port can read them, alongside your entire library.
 
 So: