feat: add syncthing debug database-statistics command (#10117)

This adds a command that shows database statistics. Currently it
requires a fork of the sqlite package to add the dbstats virtual table;
the modernc variant already has it.

This also provides the canonical mapping between folder ID and database
file, for tinkerers...

```
% ./bin/syncthing debug database-statistics
DATABASE                 FOLDER ID    TABLE                                  SIZE     FILL
========                 ====== ==    =====                                  ====     ====
main.db                  -            folders                               4 KiB    8.4 %
main.db                  -            folders_database_name                 4 KiB    6.0 %
main.db                  -            kv                                    4 KiB   41.1 %
main.db                  -            schemamigrations                      4 KiB    3.9 %
main.db                  -            sqlite_autoindex_folders_1            4 KiB    3.7 %
...
folder.0007-txpxsvyd.db  w3ejt-fn4dm  indexids                              4 KiB    1.5 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  kv                                    4 KiB    0.8 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  mtimes                              608 KiB   81.5 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  schemamigrations                      4 KiB    3.9 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  sqlite_autoindex_blocklists_1      4108 KiB   89.5 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  sqlite_autoindex_blocks_1        700020 KiB   88.1 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  sqlite_autoindex_devices_1            4 KiB    3.6 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  sqlite_autoindex_kv_1                 4 KiB    0.6 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  sqlite_schema                        12 KiB   45.9 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  sqlite_sequence                       4 KiB    1.0 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  sqlite_stat1                          4 KiB   12.2 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  sqlite_stat4                          4 KiB    0.2 %
folder.0007-txpxsvyd.db  w3ejt-fn4dm  (total)                         1906020 KiB   92.8 %
main.db + children       -            (total)                         2205888 KiB   92.0 %
```

internal/db/sqlite/db_stats.go

@@ -0,0 +1,69 @@
+// Copyright (C) 2025 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package sqlite
+
+type DatabaseStatistics struct {
+	Name     string               `json:"name"`
+	FolderID string               `json:"folderID,omitempty"`
+	Tables   []TableStatistics    `json:"tables"`
+	Total    TableStatistics      `json:"total"`
+	Children []DatabaseStatistics `json:"children,omitempty"`
+}
+
+type TableStatistics struct {
+	Name   string `json:"name,omitempty"`
+	Size   int64  `json:"size"`
+	Unused int64  `json:"unused"`
+}
+
+func (s *DB) Statistics() (*DatabaseStatistics, error) {
+	ts, total, err := s.tableStats()
+	if err != nil {
+		return nil, wrap(err)
+	}
+	ds := DatabaseStatistics{
+		Name:   s.baseName,
+		Tables: ts,
+		Total:  total,
+	}
+
+	err = s.forEachFolder(func(fdb *folderDB) error {
+		tables, total, err := fdb.tableStats()
+		if err != nil {
+			return wrap(err)
+		}
+		ds.Children = append(ds.Children, DatabaseStatistics{
+			Name:     fdb.baseName,
+			FolderID: fdb.folderID,
+			Tables:   tables,
+			Total:    total,
+		})
+		return nil
+	})
+	if err != nil {
+		return nil, wrap(err)
+	}
+
+	return &ds, nil
+}
+
+func (s *baseDB) tableStats() ([]TableStatistics, TableStatistics, error) {
+	var stats []TableStatistics
+	if err := s.stmt(`
+		SELECT name, pgsize AS size, unused FROM dbstat
+		WHERE aggregate=true
+		ORDER BY name
+	`).Select(&stats); err != nil {
+		return nil, TableStatistics{}, wrap(err)
+	}
+	var total TableStatistics
+	for _, s := range stats {
+		total.Size += s.Size
+		total.Unused += s.Unused
+	}
+	return stats, total, nil
+}