refactor(auth): remove obsolete CONVERT_PASSWORDS env var

Legacy md5/argon2id hashes are now always upgraded to bcrypt on login, so
the CONVERT_PASSWORDS flag is no longer used. Drop it from env schema,
.env.example, the docker installer, and test mocks.

src/env.ts

@@ -69,12 +69,6 @@ const schema = z
 		// Laravel APP_KEY (base64:...) — needed to read existing 2FA secrets.
 		APP_KEY: z.string().optional(),
 
-		// Deprecated: legacy md5/argon2id hashes are ALWAYS upgraded to bcrypt
-		// on login now (no flag required). Kept for config compatibility.
-		CONVERT_PASSWORDS: z
-			.string()
-			.optional()
-			.transform((v) => v === "true" || v === "1"),
 		// bcrypt cost factor used for new password hashes.
 		BCRYPT_COST: z.coerce.number().int().min(4).max(31).default(12),
 		// Filesystem dir the badge uploader writes <code>.gif into (the emulator's