Add source-specific operational docs starting with OpenSky

[calesthio]

Summary

We should add source-specific operational docs for the external data providers Crucix depends on, starting with the sources that have the most confusing failure modes and rate-limit behavior.

The immediate driver is OpenSky: public hotspot queries can return HTTP 429, and while Crucix now degrades gracefully, that behavior is not documented anywhere contributors will naturally find it.

Proposed scope

Create a docs/sources/ section with:

• docs/sources/README.md as an index / conventions page
• docs/sources/opensky.md as the first concrete source doc

If the contributor wants to extend it in the same PR, good next candidates are:

• acled.md
• telegram.md
• firms.md
• maritime.md

What each source doc should include

• What the source provides
• Auth / key requirements
• Known rate limits or instability
• What Crucix does on failure or throttle
• What the dashboard / briefing shows when degraded
• How to test it locally
• Any important caveats for contributors

OpenSky notes to capture

• Public API can return HTTP 429
• Crucix does not try to bypass rate limits
• Current behavior uses a limited generic retry via shared fetch handling
• If OpenSky still fails, source health is surfaced and the dashboard can fall back to the most recent non-empty air traffic snapshot from runs/

Why this matters

Right now this knowledge is spread across code and recent fixes. That is bad for maintainability and onboarding. A small source-doc system would make debugging, contribution, and operations a lot easier.

Good first implementation

A solid first PR would be:

1. Add docs/sources/README.md
2. Add docs/sources/opensky.md
3. Add a short README link pointing contributors to the source docs

Community contribution welcome here. If anyone wants to pick this up, comment on the issue and go for it.