fix: auto-detect platform to prevent silent data loss in Twitter-only simulations

[ChinmayShringi]

Fixes #150

Problem

When a simulation is created with Twitter-only configuration (enable_reddit=false), all data retrieval APIs silently return empty results because they default to looking up Reddit data.

Root Cause

• Platform parameter hardcoded to 'reddit' in 3+ API locations
• Twitter-only simulations never create reddit_simulation.db
• APIs look for Reddit data → empty results → user sees no data

Solution

1. Added detect_platform_from_simulation() - checks which db files exist
2. Added get_platform_with_fallback() - intelligent fallback
3. Updated 3 API endpoints to use auto-detection

Affected Endpoints

• GET /<simulation_id>/profiles
• GET /<simulation_id>/profiles/realtime
• GET /<simulation_id>/posts

Logic

• Check for actual db files: reddit_simulation.db and twitter_simulation.db
• Return platform that actually has data
• Fallback to reddit for backward compatibility

Testing

• Twitter-only simulation returns Twitter data (not empty)
• Reddit-only simulation still works
• Both-platform simulation defaults to Reddit

Impact

Eliminates silent data loss for Twitter-only simulations

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Original PR: 666ghj/MiroFish#168
Original Author: @JasonOA888