feat: Heroku vs Self-Hosting - Honest Cost-Benefit Analysis (#972)

* feat: Add Heroku to self-hosting migration guide (Issue #927)

* fix: Correct Redis backup/restore commands

* feat: Replace fake migration story with honest cost-benefit analysis

Rewrote the Heroku migration post to be transparent and educational:

- Changed from fake narrative to honest cost comparison
- Added Total Cost of Ownership (TCO) calculations with engineer time
- Included break-even analysis showing realistic 22-31% savings (not 90%)
- Created decision framework for when to self-host vs stay on Heroku
- Added 4 real-world scenario applications
- Honest discussion of hidden costs, risks, and tradeoffs
- Removed made-up performance metrics and migration story
- Focused on helping readers make informed decisions

Related to #927

* fix: Correct infrastructure savings claim in Scenario 2 (use exact calculation)

Author: bobbyonmagic

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