fix(migration): grant app_user permissions on SNMP tables

The app_user role had no INSERT/UPDATE/DELETE on credential_profiles,
snmp_profiles, or snmp_metrics — causing 'permission denied' when
creating credential profiles or SNMP profiles from the UI.

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

backend/alembic/versions/037_credential_profiles_table.py

@@ -66,6 +66,9 @@ def upgrade() -> None:
     conn.execute(
         sa.text("GRANT SELECT ON credential_profiles TO poller_user")
     )
+    conn.execute(
+        sa.text("GRANT SELECT, INSERT, UPDATE, DELETE ON credential_profiles TO app_user")
+    )
 
 
 def downgrade() -> None:

backend/alembic/versions/038_snmp_profiles_table.py

@@ -652,6 +652,9 @@ def upgrade() -> None:
     conn.execute(
         sa.text("GRANT SELECT ON snmp_profiles TO poller_user")
     )
+    conn.execute(
+        sa.text("GRANT SELECT, INSERT, UPDATE, DELETE ON snmp_profiles TO app_user")
+    )
 
     # -- Seed 6 system profiles --------------------------------------------
     for profile in SEED_PROFILES:

backend/alembic/versions/040_snmp_metrics_hypertable.py

@@ -75,6 +75,10 @@ def upgrade() -> None:
         """)
     )
 
+    conn.execute(
+        sa.text("GRANT SELECT, INSERT ON snmp_metrics TO app_user")
+    )
+
 
 def downgrade() -> None:
     op.drop_table("snmp_metrics")