Give each event its own page and show bodies the same everywhere (closes #369)

Each row of the recent events on the webhook page links to the
event's own page, /hook/{id}/events/{eventID}, and expands to show its
body; only the newest starts expanded. The event's page shows its
details, its whole body and every delivery with its attempts.

One renderer, newBodyView with templates/event_body.html, shows a body
in all three places: whole up to 32 KiB, cut there in the lists with a
link to the event's page, JSON pretty-printed, a body of more than 200
lines or 32 KiB in a scrolling box, and a body that is not text left
out beside its download link. A resubmitted copy links to its
original's page.

Model: opus-5-5

internal/handlers/event_body_view.go

@@ -0,0 +1,150 @@
+package handlers
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"io"
+	"unicode"
+	"unicode/utf8"
+)
+
+// maxRenderedBodyBytes is the most of one event's body that the
+// recent events on a webhook's page and the event log show; a larger
+// body is cut there and shown whole only on the event's own page.
+// Bodies come from the unauthenticated receiver under its 1 MB cap,
+// and renderTemplate buffers a whole page before writing it, so a list
+// of events cannot show every body whole.
+const maxRenderedBodyBytes = 32 << 10
+
+// maxInlineBodyLines is the most lines a body is shown at its full
+// height with. A body with more lines, or larger than
+// maxRenderedBodyBytes, is shown in a box of fixed height that
+// scrolls, so that it does not make the page huge.
+const maxInlineBodyLines = 200
+
+// maxIndentDepth is how deeply a JSON body's objects and arrays may
+// nest for it to be pretty-printed; a deeper one is shown as received.
+// Each level indents every line inside it two more spaces, so 10 KB of
+// nested brackets would indent to some 50 MB; within this depth a body
+// grows at most 35 times.
+const maxIndentDepth = 16
+
+// jsonIndent is the indent of a pretty-printed JSON body.
+const jsonIndent = "  "
+
+// BodyView is an event's body as the pages show it. newBodyView
+// decides it and templates/event_body.html shows it, the same way in
+// the recent events on a webhook's page, in the event log and on the
+// event's own page.
+type BodyView struct {
+	// EventURL is the event's own page. The stored body downloads
+	// from EventURL/body.
+	EventURL string
+
+	// Text is the body as shown, pretty-printed when it is JSON.
+	Text string
+
+	// Size is the stored body's size in bytes, and ShownBytes how
+	// many of them Text holds when Cut.
+	Size       int64
+	ShownBytes int
+
+	// Cut reports that Text is only the start of the body.
+	Cut bool
+
+	// Binary reports a body that is not text. It is not shown.
+	Binary bool
+
+	// Scroll reports a body to show in a box that scrolls.
+	Scroll bool
+}
+
+// newBodyView decides how to show an event's body. body is the
+// stored body, or its first maxRenderedBodyBytes when only those were
+// read, and size is the stored body's size.
+func newBodyView(eventURL string, body []byte, size int64) BodyView {
+	v := BodyView{EventURL: eventURL, Size: size}
+
+	if size > int64(len(body)) {
+		v.Cut = true
+		body = trimPartialRune(body)
+		v.ShownBytes = len(body)
+	}
+
+	// html/template shows invalid UTF-8 as replacement characters,
+	// and a browser shows a control character other than tab, line
+	// feed and carriage return as a box or not at all, so a body
+	// holding either is not text.
+	isControl := func(r rune) bool {
+		return unicode.IsControl(r) && r != '\t' && r != '\n' && r != '\r'
+	}
+
+	if !utf8.Valid(body) || bytes.IndexFunc(body, isControl) >= 0 {
+		v.Binary = true
+
+		return v
+	}
+
+	// A cut JSON document is no longer valid JSON.
+	if !v.Cut {
+		body = indentJSON(body)
+	}
+
+	// A final newline ends the last line rather than starting another.
+	lines := bytes.Count(bytes.TrimSuffix(body, []byte("\n")), []byte("\n")) + 1
+
+	v.Text = string(body)
+	v.Scroll = lines > maxInlineBodyLines || size > maxRenderedBodyBytes
+
+	return v
+}
+
+// indentJSON returns body pretty-printed when it is a JSON document,
+// and unchanged when it is not or nests deeper than maxIndentDepth.
+func indentJSON(body []byte) []byte {
+	if !json.Valid(body) || !indentFits(body) {
+		return body
+	}
+
+	var out bytes.Buffer
+
+	err := json.Indent(&out, body, "", jsonIndent)
+	if err != nil {
+		return body
+	}
+
+	return out.Bytes()
+}
+
+// indentFits reports whether the objects and arrays of the JSON
+// document body nest at most maxIndentDepth deep.
+func indentFits(body []byte) bool {
+	depth := 0
+
+	dec := json.NewDecoder(bytes.NewReader(body))
+
+	// A number too large for a float64 is still valid JSON.
+	dec.UseNumber()
+
+	for {
+		tok, err := dec.Token()
+		if errors.Is(err, io.EOF) {
+			return true
+		}
+
+		if err != nil {
+			return false
+		}
+
+		switch tok {
+		case json.Delim('{'), json.Delim('['):
+			depth++
+			if depth > maxIndentDepth {
+				return false
+			}
+		case json.Delim('}'), json.Delim(']'):
+			depth--
+		}
+	}
+}