Serve an event's full stored body over HTTP (closes #157)

The 8 KB render cap from #135 left storage untouched but no route served
the rest, so a body over the cap was reachable only with filesystem
access to the SQLite files — in a product whose purpose is storing
webhooks so they can be inspected.

GET /source/{sourceID}/logs/{eventID}/body serves the whole body to the
webhook's owner, as application/octet-stream with an attachment
disposition and nosniff. Those are a security control, not formatting:
the bytes come from the public receiver and are handed back inside the
operator's authenticated origin, and the existing CSP would not stop a
stored HTML payload executing there. The truncation marker links to it
only when a body was actually cut.

Accepted deviation, documented rather than glossed: #157's definition of
done asks the route to stream from the row. It buffers whole instead,
because database/sql exposes no incremental handle on a SQLite BLOB and
substr range reads re-materialise the entire column per call — an
earlier revision chunked at 64 KiB and was 11-15x slower for a worse
bound. Three independent reviewers confirmed no streaming path exists.

Independently reviewed three times. Two earlier revisions each asserted
a memory bound the code did not have; the final reviewer measured
2.057x at the ingest cap and pinned the two overlapping allocations from
source — the driver's column buffer and database/sql's convertAssign
clone — confirming the stated "roughly two bodies, and 2x is a floor
not a ceiling" is now accurate, since SQLite's own materialisation sits
outside the Go heap.

internal/server/routes.go

@@ -146,6 +146,15 @@ func (s *Server) setupSourceRoutes() {
 		r.Post("/edit", s.h.HandleSourceEditSubmit())
 		r.Post("/delete", s.h.HandleSourceDelete())
 		r.Get("/logs", s.h.HandleSourceLogs())
+		// The log page renders each body only up to its cap, so
+		// this is the only route that serves a whole one. It
+		// belongs to this group for its RequireAuth and
+		// NoCache; see HandleEventBodyDownload for the headers
+		// that keep the bytes it returns inert.
+		r.Get(
+			"/logs/{eventID}/body",
+			s.h.HandleEventBodyDownload(),
+		)
 		r.Post(
 			"/entrypoints",
 			s.h.HandleEntrypointCreate(),