tests: migrate assertions to testify's assert/require

Replaces raw t.Error/t.Errorf/t.Fatal/t.Fatalf across every test file
that has any (domains/accounts, domains/authentication,
domains/raw_events, domains/amazon, domains/reports x2) with testify's
assert (non-halting) / require (halting) equivalents. The three
Example-based tests (server/ui/svg, server/ui/charts) have no
*testing.T at all - nothing to convert there.

require.Eventually replaces several hand-rolled polling loops in
domains/amazon/mock_test.go. Its condition function runs on a separate
goroutine (confirmed in testify's source), so calling require.* from
inside one - which two of the new Eventually calls initially did, via
the isProcessed helper - is unsafe per Go's testing rules (t.FailNow
must only be called from the test's own goroutine). Fixed by splitting
a *testing.T-free queryIsProcessed(ctx, pool, shopID, eventID) out of
isProcessed for use inside those closures specifically.

github.com/stretchr/testify promoted from an indirect to a direct
dependency (go.mod only - it was already present transitively, so
go.sum is unchanged).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XEDaCB7C2NEBgyvqEtZuxY

go.mod

@@ -14,6 +14,7 @@ require (
 	github.com/joho/godotenv v1.5.1
 	github.com/lmittmann/tint v1.1.2
 	github.com/oapi-codegen/runtime v1.1.2
+	github.com/stretchr/testify v1.11.1
 	golang.org/x/oauth2 v0.34.0
 )
 
@@ -28,6 +29,7 @@ require (
 	github.com/bytedance/sonic v1.15.0 // indirect
 	github.com/bytedance/sonic/loader v0.5.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/dprotaso/go-yit v0.0.0-20220510233725-9ba8df137936 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.12 // indirect
 	github.com/getkin/kin-openapi v0.133.0 // indirect
@@ -60,6 +62,7 @@ require (
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
 	github.com/perimeterx/marshmallow v1.1.5 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/quic-go/qpack v0.6.0 // indirect
 	github.com/quic-go/quic-go v0.59.0 // indirect
 	github.com/speakeasy-api/jsonpath v0.6.0 // indirect