fix(server/api): preserve 500-class messages via apierror.InternalMsg

The PR1-T2 admin handler migration replaced bespoke 500-class messages
("lookup failed", "refetch failed", "trigger failed", "bump failed",
"remove failed", "update failed", "schedule row missing", "re-read
failed", "read failed") with apierror.Internal(err), which forces the
wire Message to "internal server error". That violates the PR1
contract that errEnvelope{Code, Message} must remain byte-identical
for existing clients.

Add apierror.InternalMsg(message, cause) — a 500-class constructor
that preserves the call site's user-facing message while keeping the
cause attached for logging and errors.Is. Re-migrate the 12 admin
sites whose pre-1cc7eb6 source had a non-empty bespoke Message so
they restore the original wire string. Sites whose original Message
was empty stay on Internal(err) (humanization to "internal server
error" is a tolerable improvement, not a regression).

admin_smtp.go's send_failed site (line 129) was already preserved
via a raw &apierror.Error literal in 1cc7eb6 and needs no fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/admin_coverage.go

@@ -25,7 +25,7 @@ func (h *handlers) handleGetLibraryCoverage(w http.ResponseWriter, r *http.Reque
 	q := dbq.New(h.pool)
 	row, err := q.GetAlbumCoverageRollup(r.Context())
 	if err != nil {
-		writeErrWithLog(w, h.logger, "admin: get coverage rollup", apierror.Internal(err))
+		writeErrWithLog(w, h.logger, "admin: get coverage rollup", apierror.InternalMsg("lookup failed", err))
 		return
 	}
 	writeJSON(w, http.StatusOK, coverageRollupResp{