Pivot scenario from region-based 2 users to source-based 4 users

Replaces the original APAC-vs-all 2-user demo (vpduser_a/b on
KR_ANALYSTS/GLOBAL_ADMINS groups) with a 2x2 source-access matrix:

  vpduser_my    -> MY_ONLY      group  -> MySQL view only
  vpduser_pg    -> PG_ONLY      group  -> Postgres view only
  vpduser_both  -> BOTH_SOURCES group  -> both views
  vpduser_none  -> (no group)          -> nothing (default deny)

Why: source-level segmentation is the more common production
permission story than region-level filtering. Region filtering
remains available as an opt-in variant via commented UPDATE in
sql/adb/03_seed.sql.

Key changes:
- 03_seed.sql, 07_end_users.sql, 00_cleanup.sql, .env.example,
  run.sh updated for the new 4-user model. All 4 users get
  identical view GRANTs; the only differentiator is the
  permission table (proves the model is "data-driven, not
  GRANT-driven").
- 08-11 split into one file per user: my (+ 5 bypass attempts),
  pg, both, none (default-deny verification).
- 12_tests_admin_audit.sql uses LEFT JOIN so vpduser_none shows
  up as NULL permissions, and filters by object_owner=USER to
  exclude cross-schema policies.
- Removed inline "-- comment" after ";" lines in 03_seed.sql:
  SQL*Plus silently skipped the inserts (documented gotcha).
- README.md + docs/01,02 updated for the 4-user matrix. docs/03
  detailed guide keeps the region-filter example but now has a
  preface explaining it's a variant of the default 4-user model.
- docs/04: db_type='mysql_community' note added (RDS MySQL).

E2E verified: PG=0/MY=17, PG=12/MY=0, PG=12/MY=17, PG=0/MY=0
plus all 5 bypass attempts blocked.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -22,8 +22,15 @@ export ADB_PASSWORD=""                                           # 비워두면
 
 # --- (3) 데모용 ADB 엔드유저 비밀번호 (sql/adb/07_end_users.sql 에서 사용) ---
 # ADB 비번 정책: 12자 이상, 대/소/숫자/특수 조합.
-export VPDUSER_A_PASSWORD="RowFilter#A2026"
-export VPDUSER_B_PASSWORD="RowFilter#B2026"
+# 4명의 데모 유저:
+#   vpduser_my   → MySQL view 만 SELECT 가능
+#   vpduser_pg   → Postgres view 만 SELECT 가능
+#   vpduser_both → 양쪽 view 모두 SELECT 가능
+#   vpduser_none → 양쪽 view 모두 fail-closed (0 rows)
+export VPDUSER_MY_PASSWORD="RowFilter#My2026"
+export VPDUSER_PG_PASSWORD="RowFilter#Pg2026"
+export VPDUSER_BOTH_PASSWORD="RowFilter#Both26"
+export VPDUSER_NONE_PASSWORD="RowFilter#None26"
 
 # --- (4) 원격 Postgres (AWS RDS, Cloud SQL, ...) ---
 # sql/source/postgres_setup.sql 가 여기로 customers 테이블/seed 생성.