fix(model): disable PostgreSQL prepared statements for pooler compatibility

GORM v1.25.2 closes cached prepared statements asynchronously on any SQL
error and immediately re-Parses the same deterministic name (pgx's
stmt_<sha256>) on the same client connection. Transaction-pooling proxies
(PgBouncer >=1.21 with max_prepared_statements, Neon, Supabase) respond
with FATAL "prepared statement name is already in use" (SQLSTATE 08P01)
and drop the connection. PreferSimpleProtocol only disables pgx's
implicit prepare and never covered GORM's explicit PrepareStmt cache.

- PostgreSQL now runs with PrepareStmt disabled entirely; named prepared
  statements are fundamentally session state and cannot be made safe
  under transaction pooling. Parse/plan cost is noise for this workload.
- Upgrade gorm to v1.25.12 so MySQL/SQLite statement caches (still
  enabled) no longer churn close/re-prepare on ordinary SQL errors;
  v1.25.9+ restricts eviction to driver.ErrBadConn. Deliberately not
  v1.26+, whose LRU eviction has an open use-after-close race (#7831).
- sanitizeDBError now attaches a remediation hint on 08P01/42P05 so
  affected deployments can self-diagnose from the log line.

go.mod

@@ -58,7 +58,7 @@ require (
 	gopkg.in/yaml.v3 v3.0.1
 	gorm.io/driver/mysql v1.4.3
 	gorm.io/driver/postgres v1.5.2
-	gorm.io/gorm v1.25.2
+	gorm.io/gorm v1.25.12
 )
 
 require (