Fix TypeError when revision_count is stored as TEXT

The previous schema migration added revision_count as TEXT. SQLite stores
the default as TEXT, but the app does integer arithmetic on it, causing:

  TypeError: unsupported operand type(s) for -: 'int' and 'str'

- init_db() now adds revision_count as INTEGER.
- app.py casts revision_count to int in play() and revise() and when
  incrementing in revise(), so existing TEXT values also work.

Bump version 0.4.2 -> 0.4.3.

README.md

@@ -1,6 +1,6 @@
 # Theme Song Booth
 
-**Version:** `v0.4.2`
+**Version:** `v0.4.3`
 
 A Flask web app for a convention booth where visitors request a custom AI-generated theme song, the operator manages the queue, and the final MP3(s) are delivered by email after payment.