feat: Database connection pooling

[nfebe]

Problem

internal/database/manager.go (lines ~714-718) opens a new database connection per query via sql.Open(driver, dsn) without configuring connection pool limits.

Under load, this can exhaust the database connection limit and cause query failures.

Proposed Fix

Configure pool parameters after opening:

db.SetMaxOpenConns(25)
db.SetMaxIdleConns(5)
db.SetConnMaxLifetime(5 * time.Minute)

These should be configurable via agent config or environment variables.