add PostgreSQL backend and portable backup system.

[JustRin]

What is the pull request?

Adds PostgreSQL as a second database backend alongside SQLite. Both backends are interchangeable at runtime via the panel UI or environment variables, with automatic data migration and zero data loss. Introduces a portable, driver-agnostic backup format (.xui-backup) that works with both engines. Also fixes several bugs uncovered during PostgreSQL testing.

Which part of the application is affected by the change?

• Frontend
• Backend

Type of Changes

• Bug fix
• New feature
• Refactoring
• Other

Changes

• Add SQLite/PostgreSQL switching via panel UI and env variables
• Introduce portable .xui-backup format for cross-backend backups
• Add connection pooling and PrepareStmt cache for PostgreSQL
• Fix raw SQL double-quote bug breaking queries on PostgreSQL
• Fix GORM record-not-found log spam on every Xray config poll
• Add database section to Settings with full EN/RU i18n

Details

New database layer (database/backup.go, database/manager.go, config/database.go):

• SwitchDatabase() — saves portable backup → tests target → migrates all data → writes new config
• Portable snapshot: logical JSON+ZIP export/import, driver-agnostic, supports legacy .db import
• Runtime config stored in database.json outside the panel database (or via XUI_DB_* env vars)

PostgreSQL connection (database/db.go):

• Connection pool: MaxIdleConns=5, MaxOpenConns=25, ConnMaxLifetime=30m
• PrepareStmt=true — client-side statement cache eliminates repeated parse overhead

Settings UI (web/html/settings/panel/general.html):

• Backend selector (radio buttons: SQLite / PostgreSQL)
• PostgreSQL mode: Local (panel-managed) / External — hides Host/Port in Local mode
• Test Connection / Switch Database actions
• Backup & Restore: Export Portable, Export Native SQLite, Import Backup
• Config Source row hidden in normal state; shown only for env (read-only warning)

Bug fixes:

inbound.go — raw SQL "listen = """ → parameterized "listen = ?" (PostgreSQL treats "" as identifier, not string literal)
setting.go — First() → Limit(1).Find() to suppress GORM error-level log on every Xray config poll
setting.go (controller) — testDatabaseSetting now returns correct success/failure message separately
i18n: [pages.settings.database] section — 40 keys in en_US and ru_RU

Environment variable override (Docker / IaC)

XUI_DB_DRIVER=postgres
XUI_DB_HOST=127.0.0.1
XUI_DB_PORT=5432
XUI_DB_NAME=x-ui
XUI_DB_USER=x-ui
XUI_DB_PASSWORD=secret
XUI_DB_SSLMODE=disable   # or: require, verify-ca, verify-full

When any XUI_DB_* variable is set, the Database section in the UI becomes read-only.

Screenshots