Add HTTP serve mode for Terminus/BYOS support

[ingm4r]

Summary

• Adds an optional --serve mode that starts a lightweight HTTP server (stdlib http.server, no new deps)
• Terminus Extensions can poll /data/<instance-name> for JSON instead of relying on cloud webhooks
• Adds SERVE_PORT env var for Docker, serve: config section for YAML
• Adds last_updated_local field so Terminus templates work without trmnl.user.utc_offset
• Webhook mode remains the default — fully backward compatible

Architecture

Current:  Servarr → Collector --POST JSON--> TRMNL Cloud webhook → device
New:      Servarr → Collector <--GET JSON--- Terminus Extension → device

Endpoints

Path	Response
GET /	{"instances": {"sonarr": "/data/sonarr", ...}}
GET /data/<name>	Full merge_variables payload as JSON

Files changed

File	Change
collector/trmnl_collector.py	HTTP server, store_payload(), CLI args, last_updated_local
collector/Dockerfile	SERVE_PORT env, EXPOSE 8080, entrypoint handling
collector/docker-compose.yml	Commented example for Terminus port mapping
collector/config.example.yaml	serve: section, webhook optional note
README.md	Terminus/BYOS section with link to setup guide
docs/terminus.md	Full setup guide for Terminus/BYOS users

Test plan

• Unit tested HTTP server locally (start server, store payload, GET /, GET /data/sonarr, 404 handling)
• Python syntax check passes
• End-to-end tested with Terminus: collector → HTTP serve → Terminus Extension poll → screen render
• Verify --dry-run still works
• Verify webhook mode still works unchanged
• Docker build and run with SERVE_PORT env var

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