fix: secure authentication request identity

Replace Better Auth's default trust of a caller-controlled X-Forwarded-For
with a platform-owned client-address resolution boundary: direct mode (the
safe default) trusts only the real socket peer, and an explicit trusted-proxy
mode requires a header name, a concrete trusted-proxy list, and only honors a
forwarded chain from an actual trusted peer. This closes a rate-limit bypass
where rotating a spoofed header let a caller choose a fresh bucket on every
request. Public sign-up is also disabled by default and requires an explicit
typed opt-in, using Better Auth's own disableSignUp gate so no email-existence
leak or second signup path is introduced.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

core/tests/doctor-report.test.ts

@@ -96,6 +96,8 @@ Identity
   ○ Identity configuration skipped
     Core identity schema defines 4 tables (ekklesia_core_account, ekklesia_core_session, ekklesia_core_user, ekklesia_core_verification).
     Identity runtime configuration (signing key, base URL, trusted origins) is not set.
+  ✓ Direct client-address mode
+    Trusts only the actual immediate socket peer for rate limiting; no forwarded header is trusted.
 
 Object storage
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