feat(docker): add toggle to enable/disable permanent browser instance

[mzyfree]

Summary

This PR adds a new configuration option use_permanent to the Docker deployment mode.

Problem: Currently, a permanent browser instance is always initialized at startup, which consumes roughly 200MB-300MB of memory regardless of actual usage. In resource-constrained environments or high-concurrency stress tests, this persistent instance contributes to memory accumulation and potential OOM (Out Of Memory) issues.

Solution: By introducing the use_permanent toggle, users can now choose to disable the persistent browser. When disabled, browser instances are created strictly on-demand and are fully managed by the janitor cleanup logic, ensuring better memory reclamation.

List of files changed and why

• deploy/docker/server.py: Modified the lifespan startup sequence to conditionally initialize the browser pool based on the use_permanent config. Added a fallback default of True to ensure zero breaking changes for existing users.
• deploy/docker/config.yml: Added the use_permanent flag under the crawler.pool section to make the feature discoverable and configurable.

How Has This Been Tested?

1. Default Behavior: Verified that the permanent browser still starts as usual when the config is missing or set to true.
2. On-Demand Mode: Set use_permanent: false and verified the following:
   • Log confirmation: 🚫 Permanent browser instance is disabled by default to save memory.
   • Baseline memory usage decreased significantly at container startup.
   • Successfully performed crawls; the system correctly fell back to creating on-demand instances in the COLD_POOL.
3. Resource Reclamation: Confirmed that on-demand browsers are correctly terminated by the janitor task after the idle_ttl_sec threshold is reached.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (Config updated in config.yml)
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[mzyfree]

@unclecode please review the code.