Fix reload() to always backup stats tables when stats.saving enabled

[Copilot]

Problem

When reload() is called with restat=False and no stats/counts files provided, existing stats/counts tables are not backed up to _old{n} and new empty tables are not created, causing statistics data loss.

Changes

Decouple stats table backup from stats refresh logic

• Always include stats/counts tables in swap operation when stats.saving=True
• Create empty _tmp versions if they don't exist
• Only refresh stats when restat=True

Previously, stats tables were only added to the backup list when restat=True, causing them to be excluded from the swap entirely when a user explicitly disabled stat refresh.

# Before: stats tables only backed up when restat=True
if restat and self.stats.saving:
    for table in [self.stats.counts, self.stats.stats]:
        if not self._table_exists(table + suffix):
            self._clone(table, table + suffix)
    if countsfile is None or statsfile is None:
        self.stats.refresh_stats(suffix=suffix)
    for table in [self.stats.counts, self.stats.stats]:
        if table not in tables:
            tables.append(table)

# After: backup always happens, refresh only when requested
if self.stats.saving:
    for table in [self.stats.counts, self.stats.stats]:
        if table not in tables:
            if not self._table_exists(table + suffix):
                self._clone(table, table + suffix)
            tables.append(table)
    
    if restat and (countsfile is None or statsfile is None):
        self.stats.refresh_stats(suffix=suffix)

This ensures old statistics are preserved as _old{n} and new empty tables are created during reload, regardless of whether stats are being refreshed.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Reload doesn't seem to save the old stats, and doesn't create a new empty one.</issue_title>
<issue_description>```psql
lmfdb=# SELECT
n.nspname AS schema_name,
c.relname AS table_name,
pg_size_pretty(pg_total_relation_size(c.oid)) AS total_size,
pg_size_pretty(pg_relation_size(c.oid)) AS table_heap_size,
pg_size_pretty(pg_indexes_size(c.oid)) AS indexes_size
FROM pg_class c
JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind IN ('r', 'p') -- ordinary tables + partitioned tables
AND c.relname ILIKE 'nf_fields_%'
AND n.nspname NOT IN ('pg_catalog', 'information_schema')
ORDER BY pg_total_relation_size(c.oid) DESC;
schema_name | table_name | total_size | table_heap_size | indexes_size
-------------+-------------------------+------------+-----------------+--------------
public | nf_fields_extras | 89 GB | 34 GB | 475 MB
public | nf_fields_extras_old1 | 89 GB | 34 GB | 475 MB
public | nf_fields_extra | 89 GB | 33 GB | 1245 MB
public | nf_fields_old1 | 34 GB | 15 GB | 20 GB
public | nf_fields_reflex | 459 MB | 397 MB | 61 MB
public | nf_fields_counts | 12 MB | 4624 kB | 7888 kB
public | nf_fields_oldstats | 104 kB | 8192 bytes | 0 bytes
public | nf_fields_stats | 56 kB | 16 kB | 0 bytes
public | nf_fields_extra_stats | 8192 bytes | 0 bytes | 0 bytes
public | nf_fields_extra_old1 | 8192 bytes | 0 bytes | 0 bytes
public | nf_fields_reflex_stats | 8192 bytes | 0 bytes | 0 bytes
public | nf_fields_reflex_counts | 8192 bytes | 0 bytes | 0 bytes
public | nf_fields_extra_counts | 8192 bytes | 0 bytes | 0 bytes
(13 rows)

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Reload doesn't seem to save the old stats, and doesn't create a new empty one. #47

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[edgarcosta]

This looks good to me