fix(scan): _db_complete_blocklist must return sets, not lists

B001 change caused 'unsupported operand type(s) for -: list and set' during a
full consolidated scan: _db_complete_blocklist returned split_cidr_versions()
output, which is tuple[list, list], but write_blocklist_report()/blocklist_delta()
expect set operands. Return set(v4), set(v6) to match cumulative_blocklist's
contract. Reproduced the exact report path with a populated ledger and
verified it no longer crashes.

app/scan/__init__.py

@@ -63,10 +63,11 @@ def _sync_blocklist_to_db(connected: bool, desired_v4: list[str],
 def _db_complete_blocklist(desired_v4: list[str], desired_v6: list[str]):
     """Complete cumulative blocklist from the DB ledger (B001).
 
-    Returns ``(complete_v4, complete_v6)`` derived from the active blocklist
+    Returns ``(complete_v4, complete_v6)`` as **sets** (same contract as
-    ledger plus this scan's newly desired targets (in case the DB write for
+    ``cumulative_blocklist``) derived from the active blocklist ledger plus
-    this scan has not run yet). Falls back to the git-history reconstruction
+    this scan's newly desired targets (in case the DB write for this scan has
-    when the DB is unavailable, so nothing regresses.
+    not run yet). Falls back to the git-history reconstruction when the DB is
+    unavailable, so nothing regresses.
     """
     try:
         from ..db import get_connection
@@ -77,7 +78,7 @@ def _db_complete_blocklist(desired_v4: list[str], desired_v6: list[str]):
         active = []
     union = set(active) | set(desired_v4) | set(desired_v6)
     v4, v6 = split_cidr_versions(union)
-    return v4, v6
+    return set(v4), set(v6)
 
 
 @dataclass