chore: drop the A6 rollback tombstones

A6 deployed cleanly and the provider keys are gone from the production `.env`,
which makes the legacy collector inert regardless of any settings row. The two
tombstones migration 029 pinned have no remaining job, and nothing in the
codebase reads either key.

Migration 030 deletes them and drops the Admin filter that hid them. Unlike
029's, its downgrade is meaningful — it restores both rows at their safe values,
since going back past this revision means going back toward code that reads them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

alembic/versions/030_drop_legacy_fundamentals_tombstones.py

@@ -0,0 +1,71 @@
+"""Drop the A6 rollback tombstones
+
+Revision ID: 030
+Revises: 029
+Create Date: 2026-08-07 00:00:00.000000
+
+Migration ``029`` kept two SystemSetting rows alive as rollback tombstones,
+pinned to the values a pre-A6 process needed to behave safely. A6 is deployed
+and healthy, and the provider keys are gone from the production ``.env`` — which
+makes the legacy collector inert regardless of any settings row — so the
+tombstones have no remaining job.
+
+Nothing in the current codebase reads either key.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = "030"
+down_revision: Union[str, None] = "029"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+# The safe values 029 pinned. Kept here so downgrade restores real protection
+# rather than leaving a rolled-back process reading absent rows permissively.
+_TOMBSTONES: dict[str, str] = {
+    "fundamental_data_sec_dolt_cutover_enabled": "true",
+    "job_fundamental_collector_enabled": "false",
+}
+
+_settings = sa.table(
+    "system_settings",
+    sa.column("id", sa.Integer),
+    sa.column("key", sa.String),
+    sa.column("value", sa.Text),
+    sa.column("updated_at", sa.DateTime(timezone=True)),
+)
+
+
+def upgrade() -> None:
+    conn = op.get_bind()
+    for key in _TOMBSTONES:
+        row = conn.execute(
+            sa.select(_settings.c.value).where(_settings.c.key == key)
+        ).fetchone()
+        if row is None:
+            print(f"a6_tombstone_drop {key}: absent", flush=True)
+            continue
+        print(f"a6_tombstone_drop {key}: {row[0]!r}", flush=True)
+        conn.execute(sa.delete(_settings).where(_settings.c.key == key))
+
+
+def downgrade() -> None:
+    """Restore the tombstones at their safe values.
+
+    Unlike 029's no-op downgrade, this one is meaningful: going back past this
+    revision implies going back toward code that still reads these keys.
+    """
+    conn = op.get_bind()
+    now = sa.func.now()
+    for key, pinned in _TOMBSTONES.items():
+        exists = conn.execute(
+            sa.select(_settings.c.id).where(_settings.c.key == key)
+        ).fetchone()
+        if exists is None:
+            conn.execute(
+                sa.insert(_settings).values(key=key, value=pinned, updated_at=now)
+            )