test: add invite code validation tests and fix migration rollback

- Add TestInviteCodeSignup integration tests for valid/invalid invite codes
- Fix migration downgrade: avoid dropping trigger dependency
- Add DB CHECK constraint for invite_codes.code format
- Update runtime-route.md with invite_code documentation
- Update runtime-runbook.md with change log

backend/alembic/versions/20260227_0006_invite_codes_and_profile_referral.py

@@ -20,7 +20,7 @@ def upgrade() -> None:
         """
         CREATE TABLE invite_codes (
             id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-            code VARCHAR(8) NOT NULL UNIQUE,
+            code VARCHAR(8) NOT NULL UNIQUE CHECK (code ~ '^[ABCDEFGHJKMNPQRSTUVWXYZ23456789]{8}$'),
             owner_id UUID REFERENCES profiles(id) ON DELETE SET NULL,
             status VARCHAR(20) NOT NULL DEFAULT 'active' CHECK (status IN ('active', 'disabled', 'expired')),
             used_count INTEGER NOT NULL DEFAULT 0 CHECK (used_count >= 0),
@@ -153,7 +153,6 @@ def upgrade() -> None:
 
 
 def downgrade() -> None:
-    op.execute("DROP FUNCTION IF EXISTS public.create_profile_for_new_user()")
     op.execute(
         """
         CREATE OR REPLACE FUNCTION public.create_profile_for_new_user()