Add configurable map defaults via admin settings API

[Knobee]

Summary

• Add OTS_MAP_DEFAULT_LAT, OTS_MAP_DEFAULT_LON, OTS_MAP_DEFAULT_ZOOM, OTS_MAP_DEFAULT_LAYER config settings to defaultconfig.py
• New GET/PUT /api/config endpoint with a whitelist that only exposes safe config keys (map settings initially, expandable for future settings)
• GET is available to any authenticated user (needed for map rendering), PUT is admin-only
• Defaults match current hardcoded behavior (lat 10, lon 0, zoom 3, OSM) so existing installs see no change

Test plan

• GET /api/config returns the four map settings for an authenticated user
• PUT /api/config with valid keys updates config.yml and in-memory config
• PUT /api/config with non-whitelisted keys returns 400
• Non-admin users get 403 on PUT
• Existing installs without the new keys in config.yml get defaults from defaultconfig.py

Companion UI PR: brian7704/OpenTAKServer-UI
Fixes brian7704/OpenTAKServer-UI#165

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