Remove the dead CultureMiddleware and centralize the supported-culture list

Tier 3 — cleanup

Problem

CultureMiddleware implements full cookie + Accept-Language culture negotiation but is never registered — Program.cs uses UseRequestLocalization() instead, which does the same job. The dead middleware is a maintenance trap: the supported-culture list is now duplicated in three places that can silently drift apart.

Evidence

• LearnStack/Middleware/CultureMiddleware.cs — never referenced in Program.cs
• LearnStack/Program.cs:85 — app.UseRequestLocalization()
• Duplicated culture lists: Middleware/CultureMiddleware.cs:10, Controllers/CultureController.cs:10, Extensions/LocalizationExtensions.cs:7

Proposed fix

1. Delete CultureMiddleware.cs.
2. Expose one SupportedCultures constant (in LocalizationExtensions) and reference it from CultureController.

Acceptance criteria

• One source of truth for supported cultures
• Language switching still works via cookie and Accept-Language

Start with LearnStack/Program.cs:85 and compare the culture lists in Middleware/CultureMiddleware.cs, Controllers/CultureController.cs, and Extensions/LocalizationExtensions.cs. Remove the unused middleware and centralize the list through LocalizationExtensions, then verify that cookie and Accept-Language language switching still work and only one supported-culture list remains.