Audit log retention: a way to purge, off by default

The audit log is append-only and nothing ever removed a row, so it grows for as
long as a site runs. This site reached tens of thousands of pages, which is past
the point of being useful: nobody pages through that to find anything.

Retention is OPT-IN and the default keeps everything. An upgrade that silently
started deleting a site's audit history would be indefensible, so
audit_retentiondays defaults to 0 and nothing is removed until somebody sets a
number. The Site & Facility card shows the log's size and exactly how many
entries a purge would delete BEFORE anything is pressed.

Deleting happens in committed batches. That is not an optimisation: one DELETE
over a million rows builds a single enormous transaction and holds locks across
the table while it runs, which on this server means blocking the collectors
posting inventory. The API caps its batch count so the request cannot outlive a
proxy timeout, and a run that stops early reports how much is left rather than
claiming it finished - a purge reporting success over 900k remaining rows would
be the same silent half-failure this codebase keeps getting bitten by.

`flask audit stats` and `flask audit purge` for the server, the latter reporting
by default and deleting only with --apply, with no batch cap because it is not
behind a proxy and a purge that stops halfway leaves the operator guessing.

audit.purge is a separate permission from audit.view. Reading the log and
destroying it are different acts, and plenty of people need only the first.

Chunking selects a page of ids and deletes those rather than using
DELETE ... LIMIT, which is MySQL-only while the suite runs on SQLite.

shopdb/core/models/user.py

@@ -62,6 +62,9 @@ class Permission(db.Model):
         ('users.delete', 'Delete users', 'admin'),
         # Audit
         ('audit.view', 'View audit logs', 'admin'),
+        # Deliberately separate from audit.view: reading the log and destroying
+        # history are different acts, and many people need only the first.
+        ('audit.purge', 'Delete audit log entries past the retention period', 'admin'),
         # API tokens
         ('apitokens.create', 'Create and manage API tokens', 'apitokens'),
         # Collector service tokens. A token scoped to ONLY this permission is a