Admin PUT /tenants/:id can't clear database_pool_url or database_pool_mode

[oniani1]

The PUT handler in src/http/routes/admin/tenants.ts builds tenantInfo with truthy guards:

if (databasePoolUrl) {
  tenantInfo.database_pool_url = encrypt(databasePoolUrl)
}

if (databasePoolMode) {
  tenantInfo.database_pool_mode = databasePoolMode
}

The body schema marks both as nullable: true, but a request with databasePoolUrl: null or databasePoolMode: null skips the assignment entirely. The merge upsert then leaves the existing column unchanged, so operators can't reset pool config back to the default.

The PATCH handler at lines 396-401 already does the three-way check correctly. PUT just needs the same.

Repro:

curl -X PUT 'http://localhost:5000/tenants/abc' \
  -H "apikey: $ADMIN_API_KEYS" \
  -H 'content-type: application/json' \
  -d '{
    "anonKey": "...", "databaseUrl": "...", "jwtSecret": "...", "serviceKey": "...",
    "databasePoolUrl": "postgresql://...",
    "databasePoolMode": "transaction"
  }'

curl -X PUT 'http://localhost:5000/tenants/abc' \
  -H "apikey: $ADMIN_API_KEYS" \
  -H 'content-type: application/json' \
  -d '{
    "anonKey": "...", "databaseUrl": "...", "jwtSecret": "...", "serviceKey": "...",
    "databasePoolUrl": null,
    "databasePoolMode": null
  }'

# database_pool_url and database_pool_mode are still the encrypted/string values from the first call.