perf: optimize database pools, batch commits, and node health checks

[wahh3b-lgtm]

Changes

• DB connection pool (config.py, app/db/base.py): Increased pool_recycle to 1800s, added configurable pool_timeout (default 15s) with ge=1 validation, enabled pool_use_lifo=True
• Performance indexes (Alembic migration): Added composite indexes via proper Alembic migration generated from model definitions — (status, expire), (status, used_traffic), (nodes.status), (hosts.inbound_tag), (hosts.is_disabled), (notification_reminders.user_id)
• Indexes defined in models: All new indexes are declared in __table_args__ so Alembic can detect and manage them automatically
• Record usages (app/jobs/record_usages.py): Cached dialect lookup with double-checked locking to prevent race conditions. Per-node safe_execute for node stats
• Node health checks (app/node/__init__.py): Concurrent asyncio.gather() with snapshot revalidation and exception logging for failed probes
• XRayConfig (app/core/xray.py): Reverted TLS reads to synchronous (run_until_complete on a running loop always raised RuntimeError)
• .env.example: Updated pool defaults to match new config

Files Changed

• config.py
• app/db/base.py
• app/db/__init__.py
• app/db/models.py
• app/db/migrations/versions/118c2a5eaa7d_add_performance_indexes.py (new)
• app/jobs/record_usages.py
• app/node/__init__.py
• app/core/xray.py
• .env.example

Verification

• Ruff lint + format pass
• 13/13 tests pass
• Indexes defined in SQLAlchemy models, migration generated by Alembic

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