fix(migrations): create days_to_sync_history column read by wmiau.go

[yanmayck]

Problem

wmiau.go reads days_to_sync_history when deciding how much history to request on connect:

// wmiau.go:1010
query := "SELECT COALESCE(days_to_sync_history, 0) FROM users WHERE id=$1"

But no migration creates that column.

On a fresh install the query errors out, the code logs Failed to get days_to_sync_history from database and falls through to 0, so the history-sync-by-days feature silently never runs. It only works on databases where the column arrived from somewhere else (a hand-written ALTER, or a fork that added its own migration).

Because the failure path only warns and continues, this is easy to miss — there is no error, just a feature that quietly does nothing.

Fix

Adds migration 13 — the next free ID, since 9, 10 and 12 are taken — creating the column with the same guarded pattern the surrounding migrations already use:

• PostgreSQL: DO $$ ... IF NOT EXISTS (SELECT 1 FROM information_schema.columns ...)
• SQLite: addColumnIfNotExistsSQLite(tx, "users", "days_to_sync_history", "INTEGER DEFAULT 0")

Both are no-ops where the column already exists, so this is safe to apply to existing databases.

Testing

Reviewed against the existing migration idioms in migrations.go; the SQLite path reuses the same helper as migrations 10 and 12. I don't have a Go toolchain on this machine to run the suite — worth a CI run before merging.