fix(db): use NullPool for SQLite and expose pool tuning settings

SQLite engines now use NullPool instead of QueuePool, eliminating the
"QueuePool limit of size 5 overflow 10 reached" TimeoutError under
concurrent load.  PostgreSQL/MySQL engines use a configurable QueuePool
with sensible defaults (pool_size=10, max_overflow=20) exposed via
DB_POOL_SIZE, DB_MAX_OVERFLOW, DB_POOL_TIMEOUT, DB_POOL_RECYCLE env
vars.  pool_pre_ping is enabled on all backends.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.env.demo

@@ -7,6 +7,12 @@ GOTENBERG_URL=http://gotenberg:3000
 ALLOW_FILE_DELETE=true  # Allow deletion of file records
 COMPLIANCE_ENABLED=true  # Enable compliance templates dashboard (GDPR, HIPAA, SOC 2)
 
+# **Database Connection Pool** (PostgreSQL / MySQL only; ignored for SQLite)
+# DB_POOL_SIZE=10         # Persistent connections per worker (default: 10)
+# DB_MAX_OVERFLOW=20      # Extra connections under burst (default: 20)
+# DB_POOL_TIMEOUT=30      # Seconds to wait for a pool connection (default: 30)
+# DB_POOL_RECYCLE=1800    # Recycle connections after N seconds (default: 1800)
+
 # **System Reset / Factory Reset**
 # FACTORY_RESET_ON_STARTUP=false  # Wipe all user data on every startup (demo/testing only)
 # ENABLE_FACTORY_RESET=false      # Show the System Reset page in admin UI