Move database logic into separate subdirectory

I'm trying to see if this speeds up builds a bit. May not without a
module boundary, but for now it's nice organization to have as the
program grows.

db/dberrors/history_zones.bob.go

@@ -0,0 +1,17 @@
+// Code generated by BobGen psql v0.41.1. DO NOT EDIT.
+// This file is meant to be re-generated in place and/or deleted at any time.
+
+package dberrors
+
+var HistoryZoneErrors = &historyZoneErrors{
+	ErrUniqueHistoryZonesPkey: &UniqueConstraintError{
+		schema:  "",
+		table:   "history_zones",
+		columns: []string{"objectid", "version"},
+		s:       "history_zones_pkey",
+	},
+}
+
+type historyZoneErrors struct {
+	ErrUniqueHistoryZonesPkey *UniqueConstraintError
+}