fix attachment file size limit in server backend

[a1012112796]

fix #35512

[techknowlogick]

CI fail is related:

--- FAIL: TestCreateIssueAttachment (0.49s)
    testlogger.go:61: 2025/09/22 14:12:32 modules/storage/local.go:33:NewLocalStorage() [I] Creating new Local Storage at /home/runner/work/gitea/gitea/tests/gitea-lfs-meta
    testlogger.go:61: 2025/09/22 14:12:32 HTTPRequest [I] router: completed GET /user/login for test-mock:12345, 200 OK in 2.1ms @ auth/auth.go:184(auth.SignIn)
    testlogger.go:61: 2025/09/22 14:12:32 HTTPRequest [I] router: completed POST /user/login for test-mock:12345, 303 See Other in 5.8ms @ auth/auth.go:197(auth.SignInPost)
    testlogger.go:61: 2025/09/22 14:12:32 HTTPRequest [I] router: completed POST /user2/repo1/issues/attachments for test-mock:12345, 500 Internal Server Error in 85.8ms @ repo/attachment.go:25(repo.UploadIssueAttachment)
    attachment_test.go:47: Response:  NewAttachment: attachment size 2147483648 exceed limit 104
        
    attachment_test.go:47: 
        	Error Trace:	/home/runner/work/gitea/gitea/tests/integration/integration_test.go:354
        	            				/home/runner/work/gitea/gitea/tests/integration/integration_test.go:160
        	            				/home/runner/work/gitea/gitea/tests/integration/attachment_test.go:47
        	            				/home/runner/work/gitea/gitea/tests/integration/attachment_test.go:67
        	Error:      	Not equal: 
        	            	expected: 200
        	            	actual  : 500
        	Test:       	TestCreateIssueAttachment
        	Messages:   	Request: POST /user2/repo1/issues/attachments

Also, it's probably better to have a status 413 returned if the size is too large

[lunny]

last call @go-gitea/technical-oversight-committee

[6543]

@a1012112796 -> a1012112796#13

[6543]

could push it in myselfe ...

[wxiaoguang]

No, the new changes are wrong.

header.Size can't lie.

[6543]

I try to write a prove of concept