Clean up expired sessions in SessionMiddleware #3

Summary

When the SessionMiddleware detects an expired session, it continues without the user but doesn't clean up the expired session from the database. This leads to stale session data accumulating over time.

Before You Start

⚠️ Verification Required: Please confirm this issue still exists before starting work:

1. Open internal/oauth/middleware.go
2. Find the TODO comment around line 42
3. Verify the expired session is not being deleted
4. Comment below confirming the issue exists

Current Code

// Lines 39-44 in middleware.go
// Check if session is expired
if session.ExpiresAt.Before(time.Now()) {
    // Expired session - continue without user
    // TODO: Consider cleaning up expired session here
    return next(c)
}

Expected Fix

Delete the expired session before continuing:

// Check if session is expired
if session.ExpiresAt.Before(time.Now()) {
    // Clean up expired session
    if err := storage.DeleteSession(c.Request().Context(), cookie.Value); err != nil {
        c.Logger().Errorf("Failed to delete expired session: %v", err)
    }
    return next(c)
}

Why This Matters

• Expired sessions accumulate in the database indefinitely
• Database storage grows unnecessarily
• Could impact query performance over time

Acceptance Criteria

• Call storage.DeleteSession() when expired session is detected
• Log any errors from deletion (don't fail the request)
• Add a unit test verifying cleanup is called for expired sessions
• Run tests: go test ./internal/oauth/...

Helpful Context

• The storage parameter already has DeleteSession method (see storage.go:191)
• Similar cleanup patterns exist in handlers.go (lines 896, 1206, 1299)
• Error from deletion should be logged but not block the request

Estimated Time

30-45 minutes