invoices/sql_store: switch to targeted queries, default timestamp bounds

Replace all four FilterInvoices call sites with the focused queries
introduced in the previous commit:

  FetchPendingInvoices   → FetchPendingInvoices
  InvoicesSettledSince   → FilterInvoicesBySettleIndex
  InvoicesAddedSince     → FilterInvoicesByAddIndex
  QueryInvoices          → FilterInvoicesForward / FilterInvoicesReverse

The first three are straight 1:1 swaps — the new params structs carry
only the fields that are actually used, and the removed fields (Reverse,
PendingOnly, unused index bounds) were always left at their zero values.

QueryInvoices is restructured more substantially. The forward/reverse
branch now selects between FilterInvoicesForward and
FilterInvoicesReverse, each of which takes a concrete id bound that is
always set:

  forward: AddIndexGet = IndexOffset + 1   (≥ 1 when IndexOffset = 0)
  reverse: AddIndexLet = IndexOffset - 1   (or MaxInt64 when offset = 0)

Timestamp parameters are changed from nullable (sql.NullTime with
OR-based SQL fallbacks) to always-on Go-side defaults, consistent with
the approach used by the payments query:

  createdAfter  → time.Unix(0, 0).UTC()          (epoch, before any invoice)
  createdBefore → time.Date(9999, 12, 31, …)     (far future, no upper cap)

This ensures the planner always sees plain range predicates on
created_at and can use the invoices_created_at_idx index.

The SQLInvoiceQueries interface is updated to expose the five new
methods and drop FilterInvoices.

sqldb/migrations_test.go

@@ -113,8 +113,8 @@ func testInvoiceExpiryMigration(t *testing.T, makeDB makeMigrationTestDB) {
 	// AMP invoices.
 	err = migrate(TargetVersion(4))
 
-	invoices, err := db.FilterInvoices(ctxb, sqlc.FilterInvoicesParams{
-		AddIndexGet: SQLInt64(1),
+	invoices, err := db.FilterInvoicesByAddIndex(ctxb, sqlc.FilterInvoicesByAddIndexParams{
+		AddIndexGet: 1,
 		NumLimit:    100,
 	})